Alliance are used to gain additional buffs during combat. These buffs work during sieges, duels in the league, dragon draft, battles for the Great Clan Mine or battles for the Main Castle. Most heroes belong to alliances. To receive a buff in a team, you must have the right amount of heroes from the alliance.
For example you need 4 heroes from Defenders Alliance to receive 40% buff. During the fight, active buffs are shown next to the team and these buffs are active even if one hero from the alliance set dies and lasts until the end of the round.
If a buff includes enlarging HP (like in Warriors Alliance), and is deactivated at the end of a round (because you do not have enough heroes left to form the alliance anymore), it might happen that some heroes of you die, when having not more HP then they formerly received through the buff. For example if a hero had 1000HP, got buffed a 25% with 250HP to 1250HP, is down to 200HP and the buff gets deactivated at the end of the round, it will die after the fight round has ended, because the formerly added 250HP get reduced when the buff is deactivated.
Alliance Assassins[]

All assassins receive a 50% bonus to pure damage.
2 heroes out of below hero list are needed to activate this buff.
Place above seen number of heroes out of following list in your team, to form this alliance and receive the according buff:
Goblin Assassin; Drow Fighter; XT-02/6; Mr. Zucchini; RTX-10; Grasshopper, Mr. Shark
Also the Swarm Of Flies, which has a chance to appear when the Mistress of Flies dies, is part of the Alliance Assassins.
Alliance Bards[]

All heroes receive a 15% bonus to all base stats.
2 heroes out of below hero list are needed to activate this buff.
Place above seen number of heroes out of following list in your team, to form this alliance and receive the according buff:
Garick, Divine Bard; Billy; Na'Ziba; Filvendor; King
Alliance Berserkers[]

All heroes receive a 10% bonus to ATK. All berserkers receive a 40% bonus to ATK. Berserkers can go into a rage.
2 heroes out of below hero list are needed to activate this buff.

All heroes receive a 20% bonus to ATK. All berserkers receive a 80% bonus to ATK. Berserkers can go into a rage.
4 heroes out of below hero list are needed to activate this buff.
Place above seen number of heroes out of following list in your team, to form this alliance and receive the according buff:
Troll Bigbasher; Holgeir; Tryc Staintusk; Urzag the Crusher
Alliance Blacksmiths[]

All heroes receive a +20% to ATK, +20% to DEF.
2 heroes out of below hero list are needed to activate this buff.
Place above seen number of heroes out of following list in your team, to form this alliance and receive the according buff:
Griswold; Grubbones; Alaksha, Star Smith
Alliance Defenders[]

All heroes receive a 20% bonus to defense against all damage type.
2 heroes out of below hero list are needed to activate this buff.

All heroes receive a 40% bonus to defense against all damage type.
4 heroes out of below hero list are needed to activate this buff.
Place above seen number of heroes out of following list in your team, to form this alliance and receive the according buff:
Skeleton; Isolda, Gatekeeper; Grimbeorn; Sir Zeliek; Tin Knight; Justice; Lucus; Magmatrax; Terberis; Flo; Treant; Purple Void
Alliance Dragons[]

All dragons receive +10% to DEF, +10% to WIS and a 10% bonus to defence against all damage types. All heroes receive +25% to HP.
2 heroes out of below hero list are needed to activate this buff.

All dragons receive +25% to DEF, +25% to WIS and a 25% bonus to defence against all damage types. All heroes receive +50% to HP.
4 heroes out of below hero list are needed to activate this buff.
Place above seen number of heroes out of following list in your team, to form this alliance and receive the according buff:
Dakkot, the Falling Star; Anndralgon the Black; Mecharion; Frombiss, The Hungry; Hydra; Ao Kuang; Dragon Rudolph
Alliance Druids[]

All beasts receive an additional 75% ATK, DEF, WIS, AGI. Druids can resurrect an allied beast.
3 heroes out of below hero list are needed to activate this buff.
Place above seen number of heroes out of following list in your team, to form this alliance and receive the according buff:
Newander
Alliance Hunters[]

All hunters receive a 10% to dodge. Additionally, hunters can attack an enemy.
3 heroes out of below hero list are needed to activate this buff.
Place above seen number of heroes out of following list in your team, to form this alliance and receive the according buff:
Star Hunter
Alliance Knights[]

All knights receive a 20% bonus against all damage types and a 10% bonus to additional damage.
3 heroes out of below hero list are needed to activate this buff.

All knights receive a 40% bonus against all damage types and a 20% bonus to additional damage.
6 heroes out of below hero list are needed to activate this buff.
Place above seen number of heroes out of following list in your team, to form this alliance and receive the according buff:
Vra'adin; Aurora, Angel of Hope; Raziel, Angel of Despair; Augustinus; Ophaniel; Jaguar warrior; UGX - 7b; Eagle warrior; Archangel; Ebnord peacemaker; Cossus Caderius; Titus Hiemsius; Flavius Springius; Canus Aestasius
Alliance Mages[]

All mages receive a 20% bonus to additional damage and a 15% bonus to defense against magical attacks.
3 heroes out of below hero list are needed to activate this buff.

All mages receive a 50% bonus to additional damage and a 40% bonus to defense against magical attacks.
6 heroes out of below hero list are needed to activate this buff.

All mages receive a 80% bonus to additional damage and a 65% bonus to defense against magical attacks.
9 heroes out of below hero list are needed to activate this buff.
Place above seen number of heroes out of following list in your team, to form this alliance and receive the according buff:
Zrak Damnfire; Drow Witch; Sha, Alchemist; Ethelinda; Urius, Storm Mage; Djinbo; Sicri; Tul the Fiery; Tras'tokvak; Seldana; Froghead; Malkoran; Albert; Eltor, Angel of Balance; Mistress of Flies; Frozen Elga; Zeus the Thunderer; Kuazzurk; Lord of Mictlan; Inpu, the Underworld keeper; Pyre The Spark; Irska Vess; G'eldala Zauafin; Kulazar the Decayer; Exanimus; Zegan, the Defiler; Hel; Wandering Raven;
Alliance Mechanics[]

All constructs receive an additional 75% ATK, DEF, WIS, AGI. Mechanincs can resurrect an allied construct.
3 heroes out of below hero list are needed to activate this buff.
Place above seen number of heroes out of following list in your team, to form this alliance and receive the according buff:
Mort; Tikola Nesla, Flora Liza
Alliance Monks[]

All monks deal an additional 10% damage and receive a 15% bonus to defence against all damage types. All heroes receive a 20% bonus to non magical attacks.
2 heroes out of below hero list are needed to activate this buff.
Place above seen number of heroes out of following list in your team, to form this alliance and receive the according buff:
Sun Wukong; Shenlong; Imiko; Xinyi; Gug
Alliance Paladins[]

All paladins receive a 30% bonus against all damage types. Additionally, paladins can attack the undead.
3 heroes out of below hero list are needed to activate this buff.
Place above seen number of heroes out of following list in your team, to form this alliance and receive the according buff:
Averet the Invincible
Alliance Priests[]
All heroes receive a 15% bonus to all base stats.
2 heroes out of below hero list are needed to activate this buff.

All heroes receive a 15% bonus to all base stats. Priest can heal allies.
4 heroes out of below hero list are needed to activate this buff.
Place above seen number of heroes out of following list in your team, to form this alliance and receive the according buff:
Squash; Imb; Agivot; Sluld; Loraine
Alliance Rangers[]

All heroes receive +10% to dodge.
2 heroes out of below hero list are needed to activate this buff.

All heroes receive +30% to dodge.
4 heroes out of below hero list are needed to activate this buff.
Place above seen number of heroes out of following list in your team, to form this alliance and receive the according buff:
Ena Hawkeye; G’eld Kil'Ndar; Neerd Cladd'Urden; Adie the Hawk; Kuzja; Sabwiira Demtor; Ailred; Brozk; Truzk; Star Ranger; Savad The Smuggler; Falynn Sentinel
Alliance Rogues[]

All heroes receive +20% to ATK.
2 heroes out of below hero list are needed to activate this buff.

All heroes receive +40% to ATK.
4 heroes out of below hero list are needed to activate this buff.
Place above seen number of heroes out of following list in your team, to form this alliance and receive the according buff:
Angel Eyes Stan; Ulla, the Unrelenting; Anarijel'; Queen of Pain; Sera, Queen of Thieves; Kalethah; Cabbage; Kivl; Everluar; Corgak; Zulabar; Falvieth Khazirzie; K. Lee Fly; Grunter; Quinnan Shadowblossom
Alliance Sages[]

All heroes receive +35% bonus to defense against magical attacks.
2 heroes out of below hero list are needed to activate this buff.

All heroes receive +70% bonus to defense against magical attacks.
4 heroes out of below hero list are needed to activate this buff.
Place above seen number of heroes out of following list in your team, to form this alliance and receive the according buff:
All Arbiters; Eggplant; Marlow the blue; Lennix the gray; Ulag; Ah Puch; Tagg Em; Tomato; Weed; Green Void; Tavion the Blue; Ardyll the Grovekeeper
Alliance Scouts[]

All heroes receive +5% to dodge. All scouts receive a 5% bonus to pure damage.
2 heroes out of below hero list are needed to activate this buff.

All heroes receive +15% to dodge. All scouts receive a 10% bonus to pure damage.
4 heroes out of below hero list are needed to activate this buff.
Place above seen number of heroes out of following list in your team, to form this alliance and receive the according buff:
Space Scout; Balren the Wildcrown
Alliance Shadowknights[]

All shadowknights receive a 30% bonus against all damage types. Additionally, shadowknights can attack am enemy and corrupt 70% of his HP.
3 heroes out of below hero list are needed to activate this buff.
Place above seen number of heroes out of following list in your team, to form this alliance and receive the according buff:
Ghot'gol the Crippled;
Alliance Spiders[]

All heroes receives +10% to dodge non magical attacks.
3 heroes out of below hero list are needed to activate this buff.
Place above seen number of heroes out of following list in your team, to form this alliance and receive the according buff:
Toxic Spider, Qin'qid and other Spiders
Alliance Thieves[]

All heroes receive +20% to AGI.
2 heroes out of below hero list are needed to activate this buff.

All heroes receive +40% to AGI.
4 heroes out of below hero list are needed to activate this buff.
Place above seen number of heroes out of following list in your team, to form this alliance and receive the according buff:
Skweel; Nedes the Thief; Sirius; Katelah; Tawardil; Jamis; Chani; Marras;
Alliance Troopers[]

All heroes receive +50% to ATK.
3 heroes out of below hero list are needed to activate this buff.

All heroes receive +150% to ATK, 20% bonus against all damage types, +10% to dodge.
6 heroes out of below hero list are needed to activate this buff.
Place above seen number of heroes out of following list in your team, to form this alliance and receive the according buff:
Space Trooper; Space Warrior; All Judges; Necrophagist Trooper; Necrophagist Warrior
Alliance Unicorn master[]

A properly motivated unicorn can make a lot of trouble to enemies. Bobby receives a 400% bonus to his base stats and improved combat abilities as well.
2 heroes out of below hero list are needed to activate this buff.
Place above seen number of heroes out of following list in your team, to form this alliance and receive the according buff:
Sir Abe Junior; Bobby
Note: "improved combat abilities" does not only raise Bobbys stats (400%?), it will also gain a mutli-attack on several heroes (including own, because of natural rage mode).
Alliance Wanderers[]

All wanderers receive a 30% bonus defence against magical attacks. Additionally, wanderers can attack an enemy.
3 heroes out of below hero list are needed to activate this buff.
Place above seen number of heroes out of following list in your team, to form this alliance and receive the according buff:
Star Wanderer; Necrophagist Mage
Alliance Warlords[]

All heroes receive a 15% bonus to defence against all damage types.
3 heroes out of below hero list are needed to activate this buff.
Place above seen number of heroes out of following list in your team, to form this alliance and receive the according buff:
Al Rii Lein; Zarzavat; Folium Do'Occasum; Poseidon, Lord of the Seas
Alliance Warriors[]

All warriors receive additional 10% DEF.
3 heroes out of below hero list are needed to activate this buff.

All warriors receive additional 35% DEF.
6 heroes out of below hero list are needed to activate this buff.

All warriors receive additional 75% DEF.
9 heroes out of below hero list are needed to activate this buff.
Place above seen number of heroes out of following list in your team, to form this alliance and receive the according buff:
Dwarf Miner; Abe; Sir Abe Junior; Tod; Skeleton Feralheart; Tom, Gatekeeper; Isolda; Sugkum Steelgem; Bianca; Zak, Mad Troll; King Midas; Galas, Frost Gigant; Gormmash; Helazar, Dead Lord; Mr. Pumpkin; Darthu; Brabog; Raxas Tor; Lgneath The Wraith; Dal'ek; Rakka Fireheart; Nushkilan; Carrot; Celery; Dalrur; Yagnar; Hatt the Red; Armoured Troop-Carrier; Herman von Karshtein; XT-02/6; Dendi; Sun Zi; Angmarth; McCliff; Didgur; William Vane; Lord Draador; Foyme; Morris; Prenax The Deceiver; Commander Ved'akhen; Karamal the Destroyer; Almmaharret; Kerthroraaz; Surgrazoth the Defiler; Agaxoz the Ravager; Cow King; Shale Golem; Granados, Lion King; Pumpkin Duke; Santa; Kozeentch Changing the Ways; Brallmalloch; Munnas the White; Varzoroth; Camazotz; Davy Jones; Mighty gorgon; Ninsusinak; Nirapin; Seaweed; Blue Void; Army Pumpkins; Xing-Tian; Nature Elemental; Earth Elemental; Fire Elemental; Water Elemental; Air Elemental; Odin; Ymir; Dark Jaguar (the one spawning from Jaguar warrior); Little Sphinx (The one created by Tras'tokvak)
Alliance Whisperers[]

All heroes receive +30% to dodge.
3 heroes out of below hero list are needed to activate this buff.
Place above seen number of heroes out of following list in your team, to form this alliance and receive the according buff:
Whisper; Ghost; Ufir, Ancient Demon; Lesun; Aquanis
Alliance Wizards[]

All heroes receive a 20% bonus to magic damage.
2 heroes out of below hero list are needed to activate this buff.

All heroes receive a 50% bonus to magic damage.
4 heroes out of below hero list are needed to activate this buff.
Place above seen number of heroes out of following list in your team, to form this alliance and receive the according buff:
Dragon Egg; Oona, Queen of the Forest; Kus'qudh Rotheart; Nu Wa