Bloodletting choosing its payer

Contents

Question: Who pays a Bloodletting cost, and what if it kills them?
Answer: The first eligible unit in board reading order pays all of it, and the death resolves normally.

900.20aCandidates are every living, non-Entrapped, non-boss unit on the caster's half of the battlefield. Tools, Hubs, and Pennies are all in that list. They are sorted into board reading order — left to right across each row, then top to bottom — and the first one pays.
900.20bThat is usually not the caster. Positioning the units you are willing to spend in the top-left of your board is a real decision — and a Tool or a Hub there is the worst case, not the best one. Those objects discard every combat interaction, so they cannot pay, and a payment that moves no Vitality aborts the ability before its damage.
900.20cThe whole cost is paid by one unit. It is never split.
900.20dThe payment is a Vitality cost. Armor, Insurance, Shield, and damage-ignore charges never meet it and none of them are spent by it, so a shielded payer still pays in full. A defy-death rune is the one thing that answers a lethal payment.
900.20eThe payment can be lethal. The payer dies, Macabre fires for everyone else, and the activation continues to its damage step regardless.
900.20fIf no eligible payer exists at all, the activation is aborted before damage — though its immediate rune effects still resolve.
900.20gBloodletting reduction runes lower the cost before a payer is chosen. Cards that read "the cost paid" use the Vitality that actually left the payer — the reduced cost, or the payer's remaining Vitality, whichever is smaller.