Mari, the Killing Quill is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Whenever a creature an opponent controls dies, exile it with a hit counter on it. Assassins, Mercenaries, and Rogues you control have deathtouch and "Whenever this creature deals combat damage to a player, you may remove a hit counter from a card that player owns in exile. If you do, draw a card and create two Treasure tokens."

Complete guide

Mari, the Killing Quill is a powerful Legendary Vampire Assassin that turns your opponents' fallen creatures into valuable bounties. By marking exiled cards with hit counters, she provides a continuous resource engine for your tribal deck. This card effectively punishes opponents for losing creatures while rewarding your aggressive plays with card draw and mana.

Marking the Target

Mari's first ability is a triggered ability that looks for any creature an opponent controls dying. Instead of staying in the graveyard, those creatures are exiled with a hit counter. This is not a replacement effect, so 'dies' triggers from other cards will still fire before the card is moved to exile. This ability ensures that every time an opponent's creature is destroyed or sacrificed, it becomes a potential source of loot for your Assassins, Mercenaries, and Rogues.

Cashing in the Bounty

Mari grants three specific creature types—Assassins, Mercenaries, and Rogues—deathtouch and a combat damage trigger. When one of these creatures deals combat damage to a player, you can choose to remove a hit counter from a card that player owns in exile. If you successfully remove the counter, you draw a card and create two Treasure tokens. Note that the card must be in exile and must have a hit counter; you cannot remove counters from cards in other zones or those without hit counters.

Strategic Timing and Common Mistakes

One common mistake is forgetting that the hit counter must be removed from a card owned by the player who was dealt damage. If Player A controls a creature in exile with a hit counter but you deal combat damage to Player B, you cannot remove Player A's counter to get the bonus. Additionally, because the exile ability is a trigger and not a replacement effect, opponents can still benefit from 'when this creature dies' effects before Mari exiles the card.

Interaction examples

Bypassing the Graveyard

Solemn Simulacrum

If your opponent controls Solemn Simulacrum and it dies, its own ability triggers along with Mari's. The opponent will draw a card from the Simulacrum's ability, and then Mari's ability resolves, exiling the Simulacrum from the graveyard and putting a hit counter on it.

Doubling the Payout

Roaming Throne

If Roaming Throne enters the battlefield naming 'Assassin,' it gains the Assassin type and benefits from Mari's deathtouch. Furthermore, Roaming Throne causes Mari's combat damage trigger to fire an additional time. If you have two cards in exile with hit counters owned by the player you hit, you can remove both to draw two cards and create four Treasure tokens.

Generating Wealth

Treasure

After an Assassin deals combat damage to a player, you remove a hit counter from their exiled creature. You immediately draw a card and create two Treasure tokens. These Treasure tokens can be sacrificed at any time to add one mana of any color, effectively paying for Mari's mana cost in just one successful attack.

Guide written with AI from the official text and rulings. Official English text prevails.