Crypt Masters

The Crypt Masters of Mechanicsburg are very old, probably Constructs and protect the children of the town during battles, somewhere beneath the Red Cathedral. Presumably in the crypts, which they presumably also take care of at other times. They speak in the SSPEECH OF VOWING, even when not making vows.

They're pretty impossible to visually distinguish, so it's unclear whether the one Tweedle killed on the way to the Mirror was the same one who was present with Vanamonde's Inner Council. It's also unclear whether he is a member of that Council.

Possibly relevant outside information
In Terry Pratchett's novels, Death speaks in ALL CAPITALS, much like the Speech of Vowing. Given that he's in their avowed inspirations and has a shout-out, that's probably not a coincidence.