AB-21 was a Warforged transmuter who was part of a four-Warforged team who oversaw the Northern Terminus along with some Worker constructs.

In the aftermath of the Ytarran extinction, he and his colleagues began to break down due to lack of repair and eventually MN-8 and CC-1 died of "old age". TR-4 and AB-21, however, were not content to go gentle into that good night and began to cannibalise parts from the other Warforged in order to prolong their own lives. After AB-21 died, TR-4 even managed to find a way to jury-rig incompatible parts, but this was of couse too late for the poor abjurer.

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