Alliances are needed to play against the top tiers of players. Alliances give support by means of other players and shrine bonuses for the whole alliance.
Creating and joining an alliance[]
https://forums.kingsoftherealm.com/showthread.php?714-Creating-and-Joining-Alliances&p=2924#post2924 Dunwho (a dev) posted a good guide for accepting and joining an alliance, it also has pictures.