Valley of Glithildhoul
From HGWiki
More Lair Guardians and the dragon Glithildhoul (level 40 accomplishment (see Accomplishments)). This dragon drops one of the two seals you will need to enter the lair of the Mother of all Dragons. This dragon is NOT vulnerable to Isaac's Greater Missile Storm so you'll have to find other ways of killing it. Rebuke and Vortex of Teeth function but in most cases you will need some melee or ranged damage to speed up the kill. Divine Slingers are very effective for this as they can deal significant Divine or (if pure) Magic damage.
Rest Spawn Chance
Zero
Creatures
- Lair Guardian (both melee and ranged)
- Glithildhoul