The idea in general across the line was that the Ultraverse numbers were falling precipitously, and so connecting it to the Marvel Universe as rapidly and as often as possible was a mandate. This is why there were suddenly something like a dozen Marvel/Ultraverse crossovers that were hastily assembled. In terms of the Black Knight and Sersi, they were coming out of AVENGERS and so were at loose ends enough to be drafted into the Ultraverse books without wrecking up anybody’s plans.