When Dream Seizer dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
When Dream Seizer enters the battlefield, it triggers, putting a -1/-1 counter on it.
Resolve the Shilgengar ability
Repeat.
This clever loop transforms Shilgengar, Sire of Famine and Dream Seizer into an endless sacrifice engine, fueled entirely by the passive blessing of Mikaeus, the Unhallowed. When you feed Dream Seizer to Shilgengar, Mikaeus grants the insect horror undying, bringing it straight back to the battlefield with a +1/+1 counter to offset its recurring -1/-1 counter from blighting. This endless cycle of death and rebirth lets you continuously sacrifice Dream Seizer to generate an infinite supply of Blood tokens, which can eventually be cracked to resurrect your entire graveyard.