Should have uploaded these yesterday, sorry 
EDIT: And here is a save juste before all of this happens. Check Hannis army near the Fissure. You'll get Procipinee warning just prior to be allowed to attack.
https://www.dropbox.com/s/bxz4tannfwchrn7/bug%20Albeix-before.EleSav
Well, if this is the case, why does The Lord who Dwells Beneath appears on the strategic map, on the combat initiation screen AND the victory screen with several other spiders, BUT only one rock spider spawns on the battlefield ?
My guess would be it is caused by this (very) special map...
(Oh, and if it was possible to just enlarge the passage to Albeix spawn point by a tile or two, it would be much more convenient to deploy troops... defenders always seems to spawn on the very rear 

Looks like Albeix himself, right ?
REALLY looks like him...
...this screenshot was taken at my first turn. Turns out the Big Worm was a little spider after all ?

Hmmm, what ? except the level 4 spider I did NOT see any of these beasts...