Red Skull was sent over the Shield to die in the wastelands after leading a failed insurrection against Doom. Even though nobody’s seen him since and no-one survives the wastelands for long, Doom wants Crossbones to lead a group of supervillains - there’s half a dozen picked out but only Magneto is relevant - over the Shield to find and bring him Red Skull’s head in a box.
This Secret Wars tie-in was completely pointless. Nobody was asking what Red Skull was up to during Secret Wars (not much as it turns out), his plan to topple the Shield and fight Doom again was stupid and irrelevant (especially given that it falls to someone else), and at no point is any of it in the least bit entertaining. I liked that the supervillain burg of Battleworld is called Killville though.
Full of crap art and even worse writing, if you want to read a slightly better version of a very similar storyline, check out the Planet Hulk tie-in. Red Skull is red shit.
Battleworld: Red Skull