If a permanent entering causes a triggered ability of a permanent you control to trigger, that ability triggers an additional time.
Yarok doubles all your enters-the-battlefield triggers, turning value creatures and lands into engines that snowball card advantage and board presence. You ramp into Yarok, then chain blink and recursion effects to draw, ramp, drain, and grind opponents out of resources before closing with a critical mass of triggers or a combo. Most games are won by overwhelming value or an infinite ETB loop.
Doubles every ETB trigger, massively amplifying value creatures and engines
Deathtouch + lifelink make it a respectable defensive blocker and life buffer
Sultai colors give access to the best ramp, card draw, removal, and tutors
Enables several compact infinite combos with cheap ETB pieces
Heavily commander-dependent; the deck slows dramatically without Yarok out
High curve and value pieces can be slow against fast combo/aggro decks
Vulnerable to graveyard hate and 'enters tapped/can't draw' stax effects
Five-mana commander invites removal before you generate value
Copies Yarok to dodge legend rule via blink shenanigans or copies your best engine.
Add fast mana (Mana Crypt, Sol Ring, Chrome Mox) and efficient tutors (Demonic Tutor, Vampiric Tutor, Worldly Tutor) to find Yarok and combo pieces faster. Tighten toward compact two-card infinite combos (Cauldron Familiar lines, Karmic Guide loops) and protect Yarok with counterspells and recursion. Trim clunky high-cost value creatures for low-curve ETB enablers and interaction to keep pace with faster tables.