Probably the Merlin, told by Luccio that he is Harry’s brother, setting up Harry to be killed by Eb using Thomas as bait. The Merlin doesn’t know Eb is their grandfather. It would be a very Merlin thing, he tried something similar with Morgan, but no one else really wants to take on Harry anymore amongst the wardens. The Merlin would be delighted if Harry took out Eb in the process whether Harry survived it or not.
Of course Mab is behind it to remove Langtrey as Merlin/cause the formation of the Grey Council as a force under the accords. One of the favours owed to Mab is the knowledge via Justine that Luccio knows about Thomas (but doesn’t know about Eb)
