I've also seen him spawn as a wanderer in between the 2nd and 3rd rooms as you move in. The second room being the one with regular sewer trash like slime, bugs, turtles, and the 3rd one with the undead in it.
He dropped the spell: KEI, the Yikta rune, the timing gear and the Moss Stitched Gloves all at once for us.
Cryxtetl spawns several rooms before the "furnace room", in the "two floor" chamber with water on the bottom, and the broken sotne ramp going up the center. As far as I know, he spawns in the water at the base of the broken ramp (My group was one room ahead of this place when he showed up on tracking, we doubled back for him.)
A ghost mob, he dropped the Timing Gear like most other nameds in the zone, and the +15 ATK Augment Scoundrelx references. I forget the name. He/she also was casting spells, but nothing too destructive.