Book of Lairs is an adventure for four 5th-level characters. The OGL content is limited to the monster statistics which mostly come from Tome of Beasts and the Monster Manual. A compact in-line stat block is used for monsters.
OGL Content:
Bone Crab - (monster)
Fraughashar - (monster)
Giant, Thursir - (monster)
Hobgoblin Captain - (monster)
Orog - (monster)
Putrid Haunt - (monster)
Rime Worm, Adult - (monster)
Rime Worm Grub - (monster)
Rime Worm Larval Swarm - (monster)
Rime Worm Larval Swarm, Venomous - (monster)
Spark - (monster)
Yeti - (monster)