When Dread Slaver enters the battlefield, Be'lakor's second ability triggers, causing Dread Slaver to deal damage equal to its power to itself.
Activate Ashnod's Altar by sacrificing Dread Slaver, adding CC.
When Dread Slaver dies, it triggers, returning itself from your graveyard to the battlefield.
Repeat from step 2.
By placing Maskwood Nexus on the battlefield, you fundamentally rewrite the rules of your army, making every creature you control—including Dread Slaver—a Demon in addition to its other types. This type-shifting transforms Be'lakor, the Dark Master into a relentless engine of self-destruction; whenever Dread Slaver re-enters, Be'lakor's trigger forces it to deal damage equal to its own power directly to itself. That fatal damage sets up Dread Slaver's own recursion ability to pull itself straight back from the graveyard, while Ashnod's Altar stands ready to harvest that sacrifice loop for an endless surplus of colorless mana.