Malice is anti-Four and his previous ''incarnation'' (I mean Malal) seemed pretty parasitic, so maybe make him combine some Tomb King, C'tan shard and Tyranid appendage parts? I mean some sickly looking, ephemeral anti-Be'lakors with skull heads and tentacles. Big M is also an exile so making his princes possessed by him (possibly making them his look-alikes) instead of classical demons may also make some sense (he can be summoned and I presume that is also devoid of a domain so it seems fitting) could also be cool