On Easter Sunday evening, in a palatial house on a ridge high in the hills above Hollywood, Offset puffs on a joint as songs from his forthcoming album blast from a portable speaker. He’s been working on this album for many months — since Migos, the pioneering Atlanta hip-hop trio with which he rocketed to stardom in the mid-2010s, drifted into an unofficial hiatus a couple of years ago, and well before the shocking, tragic murder of his bandmate Takeoff outside a Houston bowling alley on Nov. 1. Several members of Offset’s team mill around the sparsely furnished house — one of three residences for the rapper and his wife, Cardi B — while the man himself, casually clad all in black, bobs in his seat, his wildcat eyes sparkling.