Deepfathom Skulker

The two halves of this card answer each other, and that closed loop is the whole reason it works. The trigger turns combat damage into cards, which is generous but conditional: it pays out only if your creatures connect, and connecting is exactly what a board stall denies you. The activated ability supplies the missing piece, manufacturing an unblockable attacker on demand so the draw engine never goes dark. Spend the colorless mana, push one creature through, draw a card, repeat. Most card-advantage finishers ask you to win the combat math first and reward you for it; this one fixes the math itself, which is rarer and more self-sufficient.

Devoid is the structural wrinkle worth noting, though it does less than it first appears. It strips the printed color so the creature reads as colorless on the battlefield, which matters for spells and effects that key off color (anything checking for blue creatures, or color-specific protection and targeting). What it does not touch is color identity: the in the cost keeps this a blue card wherever identity is the relevant test. The in the unblockable ability reinforces that the Eldrazi were built to lean on a different resource than generic mana, asking for true colorless to fuel the push. The result is an attrition engine dressed as a beater: a 4/4 body that, given enough mana, refuses to stop refilling your hand as long as anything you control keeps swinging.