Without any real ideas, I'd just like to point out that a kruthik lair might be a curve ball. While doing something else, the PCs stumble upon the Kruthik lair.

They could be doing pretty much anything that is related to your campaign plot which you wish delayed or even enriched by the findings in the kruthik lair.

For example:
While in search for a forgotten mountain pathway from which to contact a northern city more easily (or prevent a goblin raid from that direction, or encircle an enemy or unicorns) the PCs suddenly find themselves in the middle of a hill landslide (could be a fun skill challenge). Of course, they soon find themselves in underground tunnels made by Kruthik without much hope of going out through the hole they fell in, which, oh my, is maybe even sealed with dirt!
So they go around trying to figure out how to get out, and, in the end, find something which benefits their previous plan.

Basically, you don't need a reason for them to encounter a full kruthik lair, you just need a reason to get them to do anything. :)

Hope it helped, cheers!