The five primary gods of Theros are known to wield mystical weapons with fabulous capabilities, enhancing their rule over their respective domains. So far, three of them - Thassa's bident Dekella, Purphoros's hammer Akmon, and Nylea's bow Ephixis - have had their cards spoiled. That of Erebos, the god of the underworld - Mastix, the whip that captures wayward souls - is yet unknown. But we can still guess at what it might do.
In the Planeswalker's Guide to Theros, Part 1, it's described thusly:
Erebos wields Mastix, a golden-handled whip with an impossibly long lash, like a thin shadow that stretches on forever. The whip is a means of inflicting pain when he must, but its more frequent function is as a snare to pull the reluctant dead into—or back into—his realm.
All of the weapons are legendary enchantment artifacts that have mana costs of their masters' respective colors. They also all have one static ability and one activated ability that costs mana plus a tap. Now, we know Erebos's purview: He's the god of wealth but especially of death and the dead, and can allow the souls of the forsaken to leave his realm for a second chance, and steal the life essence ot mortals. It should be easy to make a resonant, flavorful Whip that captures those things:
Whip of Erebos 1BB
Legendary Enchantment Artifact (Rare)
Creatures you control have lifelink.
5B, T: Each player may return a creature he or she controls from his or her graveyard to the battlefield.
What do you think Mastix will be like? Let me know in the comments.