They didn't. At least not yet...
spoilers:end of spoilers
Mordred infiltrated New Attilan and used his magics to make Ulysses show him a potential future where Namor helps Warrior Woman conquer earth with the combined forces of the Myriad & Atlantis. Zarda finds this interesting since they recently killed Namor. Mordred surmises that since so many heroes have been brought back from the dead, they shouldn't have trouble bringing Namor back.