"The blood shall be a sign for you on the houses where you live; and when I see the blood I will pass over you, and no plague will befall you to destroy you when I strike the land..."
The Lair of Gutslime is a lair hidden in an area of Mount Bloodhorn, northeast of the Greenskin starting area in a wooded area called Felridge. Gutslime's Lair is hidden within the eastern mountains, and hopping over a few small gaps will reveal a small cave tucked away behind foliage. When you discover the area, you will get a Tome of Knowledge unlock.
Killed Nerx Gutslime drops 36 level rare (blue) weapons
Felridge is the area northeast of the Greenskin starting area, and can be accessed by heading north though Da Gobbo Camp and traveling east along the trail past Old Stoneson Mine, then south and over the bridge to Felridge. Once there, travel southeast along the northern mountain line until you see a broken Dwarf statue with a fire burning on its head. You'll spy a conspicuous looking wooden plank if you turn left at the statue, and hopping over a gap and climbing the plank, then turning left and hopping over another small gap will lead you to Gutslime's cave.
Just go to the back of the canyon where the Marghaz Bloodtoof Ritual PQ is. Easy to find.
Approximate Location: 53000,45000