When Witch-king dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
Activate Woe Strider by sacrificing Witch-king
When Witch-king dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
Resolve both Woe Strider abilities from step 1 and 3.
Repeat.
This loop relies on seamlessly alternating between the two sides of Eirdu, Carrier of Dawn // Isilu, Carrier of Twilight to constantly flip the destiny of Witch-king, Sky Scourge. With Isilu granting persist and Woe Strider acting as a reliable sacrifice outlet, Witch-king can repeatedly cycle between the graveyard and the battlefield, with its undying and persist counters perpetually canceling each other out. Meanwhile, Woe Strider brings along a Goat token and lets you scry through your deck every time you sacrifice, turning this cyclical resurrection into a deep well of triggered abilities while fueling your graveyard.