"Pleased to say that I'm leaving a great franchise for another awesome one in Halo," DelHoyo revealed on Twitter. "Going to Seattle to join the multiplayer team at 343."
DelHoyo had been responsible for the multiplayer in "Gears of War 3" and the upcoming "Gears of War: Judgment". Specifically, the new OverRun mode which mashes up Horde mode and Beast mode from "Gears of War 3" was his baby and designed with eSports in mind.
That last bit is an important distinction as 343 Industries and Microsoft signed with Virgin Gaming for competitive eSports which appeared to be the reason "Halo 4" was not picked up by MLG. While there's probably not much he can do with the current "Halo" beyond working on the upcoming competitive playlist, he should make an impact on "Halo 5".
DelHoyo's departure continues a string of key Epic Games personnel leaving the company over the last year. The list include Cliff Bleszinksi to parts yet decided, Rod Ferguson to Irrational Games, Mike Capps to retirement, and a creative lead and two artists from People Can Fly following Epic Games buying the developer.