This type of monster is found in Guardstone in the Bloodlust part of the map. They are found in a Tomb-like building in the far Northwest to the map (a bit more Southeast in relation to Bloodlust's base), near where level 1 Grubs and level 2 Green Grubs are. It is possible that Ghastly Grubs are the replacement of Ghastlings, but with a more advanced level and strenght.
Tips
Ghastlings are the secondary early way to farm near Bloodlust's base, if you don't want to farm (Mature) Grubs or they're already being defeated by other players, this is a good option. There are only 3 of them, so the problem won't be THAT big.
Ghastlings are more advanced than those Grubs, but can be defeated with your first spell (like Ice Bolt)
Gallery.
Trivia
This is one of the mobs that never appeared with a Yellow-colored level, alongside Grub Younglings, level 1 Grubs, level 2 Green Grubs (all found in Guardstone), Gnome Archers, Gnome Kings (both found in Headless), level 2 Cyan Grubs and level 2 Mature Grubs (both found in Guardstone, too).
