Waning Wurm 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

Vanishing 2 (This creature enters with two time counters on it. At the beginning of your upkeep, remove a time counter from it. When the last is removed, sacrifice it.)

Complete guide

Waning Wurm is a massive Zombie Wurm that offers a high power-to-mana ratio at the cost of its own mortality. Thanks to its Vanishing ability, it provides a brief but devastating presence on the battlefield before it inevitably rots away.

What it Does

Waning Wurm is a 7/6 creature for only four mana, but it is bound by the Vanishing mechanic. It enters the battlefield with two time counters, and these counters act as a countdown clock for how long it can remain in play. If you cannot win or find a way to manipulate the counters, it will be sacrificed relatively quickly.

How it Resolves Step by Step

When Waning Wurm enters the battlefield, it immediately gains two time counters. At the start of your Upkeep, a triggered ability goes on the stack to remove one time counter. If that was the very last time counter, a second 'state-trigger' immediately causes you to sacrifice the Waning Wurm. This means that without intervention, it will usually be able to attack exactly once.

Common Mistakes

A common point of confusion is the difference between Vanishing and Fading. With Vanishing, the creature is sacrificed when the last counter is removed, not when you fail to remove one. Additionally, note that if you have multiple permanents with Vanishing, each one triggers its own counter removal separately during your upkeep.

When to Play it

Waning Wurm is best utilized in aggressive decks that need to close out the game quickly or in decks that can capitalize on high-power creatures. It also pairs well with cards that can 'proliferate' to add more counters or effects that prevent the sacrifice trigger from ending its life.

Interaction examples

The standard countdown

You cast Waning Wurm on turn 4. It enters with two counters. On your turn 5 upkeep, it goes to one counter; you then attack for 7 damage. On your turn 6 upkeep, you remove the final counter and must sacrifice the Wurm.

Maximum impact

If you give Waning Wurm Haste the turn it enters, you can attack for 7 damage immediately. By the time the second upkeep trigger removes the last counter, you will have potentially dealt 14 damage before the creature is sacrificed.

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