Centruo

Did some checking and it appears to be the wurm pet causing the problem with the illusionist passive, rather than the summon. Didn't trigger without the pet equipped.

When playing as a Loremaster, specifically a Beckoner/Illusionist, it seems the Illusionist passive is passed on to the wurms (after using Gernisc Slew the Beast, But Soon Faced Its Kin,) causing a huge number of non-aggro/non-despawning wurms to appear on the screen, eventually to the point of crashing the game. The wurms don't attack anything, but they will break stealth if you cross their path.