Darksteel Reactor 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

Indestructible (Effects that say "destroy" don't destroy this artifact.) At the beginning of your upkeep, you may put a charge counter on this artifact. When this artifact has twenty or more charge counters on it, you win the game.

Complete guide

Darksteel Reactor is a patient and resilient win condition that rewards players for playing the long game. Because it is indestructible, your opponents will need more than a simple Shatter to stop your inevitable victory. As long as you can protect yourself, this artifact will eventually end the match in your favor.

A Slow But Steady Clock

Darksteel Reactor functions through a triggered ability that occurs at the beginning of your upkeep. You have the option to put a charge counter on it every turn, but you are not forced to if you have a specific reason to stay at your current count. Because it is indestructible, it ignores 'destroy' effects and lethal damage, meaning most common removal spells will simply bounce off its metallic surface.

The Path to Victory

The game-winning ability is a state-triggered ability. This means that as soon as the twenty-counter threshold is met, the ability triggers and goes on the stack. You do not have to wait for your next upkeep to win; if you find a way to add counters during an opponent's turn or during your main phase, the win condition will trigger immediately once those counters are placed.

Common Mistakes

Players often forget that 'Indestructible' does not mean 'Immune to everything.' Darksteel Reactor can still be exiled, returned to your hand, or sacrificed if an effect instructs you to do so. Additionally, if the Reactor is removed from the battlefield in response to its own winning trigger, you will still win the game because the ability has already been placed on the stack.

Strategy and Timing

Since the win condition requires twenty turns by default, you should focus on defensive play. Deploying this artifact early allows it to start accumulating counters while you build your mana base. It is a 'must-answer' threat that forces your opponents to find specific non-destructive removal, often distracting them from your other plays.

Interaction examples

Early Game Setup

Solemn Simulacrum

You cast Darksteel Reactor on turn four. On turn five, you cast Solemn Simulacrum to search for a basic land, ensuring you have the mana to cast defensive spells. During your next upkeep, the Reactor gains its first counter. The Simulacrum provides a blocker that draws you a card when it dies, helping you survive the twenty turns needed for the Reactor to reach its goal.

Maintaining Resources

Commander's Sphere

With Darksteel Reactor on the battlefield, you use Commander's Sphere to provide the mana needed for counter-magic or removal. If an opponent tries to use a spell to exile your Reactor, you can sacrifice the Commander's Sphere in response to draw a card, digging for a way to protect your win condition or find a new threat.

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