Ureni of the Unwritten - Intelligence

Ureni matters when the deck can cast a seven-mana Spirit Dragon, keep enough Dragon creature hits in the deck, and turn each enter or attack trigger into the best Dragon from the top eight cards.

Each Ureni trigger can put up to one Dragon creature card from the top eight onto the battlefield.

The chosen Dragon enters from the library without being cast, so cast triggers and alternate casting costs do not happen.

Removing Ureni after its enter or attack trigger is on the stack does not normally remove that trigger.

Ureni's attack trigger happens when it is declared as an attacker, not when it deals combat damage.

Haste can let Ureni enter and attack in the same turn, creating both an enter trigger and an attack trigger.

Additional combat phases can create additional Ureni attack triggers if Ureni attacks again.

Cards not put onto the battlefield are put on the bottom of the library in a random order.

Seven-mana commander tax, removal, counters, low Dragon density, excessive top-end, wipes, Torpor Orb effects, fogs, and graveyard hate are the main failure points.

Use land ramp and fixing to cast Ureni, then keep Dragon density high enough that the top-eight trigger finds a meaningful creature.

Give Ureni haste so the deck can get an enter trigger and an attack trigger in the same turn.

Blink or recur Ureni to reuse the enter trigger, then prioritize Dragons with ETB damage, copy, token, or mana text.

Use attack-trigger Dragons and combat-phase engines to create more Ureni attack triggers.

Use Scroll Rack, Brainstorm, Sylvan Library, Worldly Tutor, and Noxious Revival to place the right Dragon where Ureni can find it.