From Lexicanum

Extremis Diabolus is the most serious accusation that an Inquisitor can possibly give. It is used most often by Inquisitors of the Ordo Malleus for consorting with being under the influence of daemons. Witchcraft, practicing forbidden arts and heresy against the Emperor are all grounds for pronouncement of Extremis Diabolus. The punishment is death.

Sources

• The Inquisition p34