The answer came easily.
Beginning.
After the Undead High Drake, it could be argued that Beginning was the strongest undead in Michael's legion.
He had considered the Bone Dragon first. After all, if the effects of Echoed Bestowal truly pulled creatures closer to the origin of their bloodline depending on the chosen traits, then an undead like the Bone Dragon should receive an enormous boost with Beginning's and the High Drake's traits fused to it.
However, putting aside compatibility, the Bone Dragon could not even defeat Beginning under normal circumstances, which made Michael lean heavily toward the giant.
Beginning was stronger than nearly every other undead in his legion, and its potential ceiling could easily rank among the top five. Its body was absurdly powerful. Its Law of Brutal Rebirth made it increasingly difficult to kill. Its instincts in battle were terrifying.
If any undead had the qualifications to temporarily step into the Demigod level, it was Beginning.
