Caldarius (click for map image) spawns in the easternmost room of the second floor, in the Anetarx Extraction Lab: PH "a worn arc worker". PH respawn time is 4 min 50 sec.
Map point for "arxmentis.txt" map files (for all Arx Mentis named map points, see this quest entry):
Hits for a max ~31,000; has ~13.5 million hitpoints.