no there is not any whay to join them :( you can not even get positive relaionship with them :) but you can have some heretic troops, best of them are heretic magnus and Doomgide, you can not have undead heretic units in party like demon magnus :(. there is much to do if you like to have lot of them in party, i can write you plan for it.

1. there must be seer army and inquisitor in pendor.
2. bring seer army near inquisitor, let them fight.
3. Do not rush in battle let inquisitors fight to lose some troops vs seers
4. after you see inquisitors have so little army you can beat it with your troops join on there side
and win battle.
5. after battle kill inquisitors troops and get thous seer units at your party :) take Seer Favorite and Seer Initiate
6. update them to Doomguides

You can only join the corrupt order of Eventide, they are in league with heretics and are good knights, but you cant join the heretics.

How do I join Eventide?

Originally posted by Handlanger:

You can only join the corrupt order of Eventide, they are in league with heretics and are good knights, but you cant join the heretics.

The first thing you need to do is increase your relations with them, you will start the game with -40, so in order to increase your relation you need to assist their parties in fights, you just need to wait in Singal for a party of these knights spawn, they will spawn 15 or 30 troops, you must have at least 40 good men, them you must lure them to the north of Fierdsvain coast and assist them fight against the vanskerries, once you get 0or higher relation, you will be able to do quests for the order, to join them you will need 500 renown and at least 15 or 20 honor. But keep in mind that if you want to create a chapter of this order you will need Sir Alistair and -22 honor!

They Chapterhouse is currently in Singal, in the Dshar lands.

you can bring them at chapter Snake Cult Rituals or Heretic Writings, ech one of them will gave you +1 relaionship. also Order of Eventide is not in top 5 orders.