Activate Ashnod's Altar by sacrificing Neriv, adding CC.
When Neriv dies, Nim Deathmantle triggers, causing you to pay 4 to return Neriv from your graveyard to the battlefield and attach Nim Deathmantle to it.
When Neriv enters, it triggers, creating two creature tokens.
Activate Ashnod's Altar by sacrificing a creature token, adding CC.