Duergar Mine-Captain — ruling 1

This ability affects only creatures that are attacking at the time it resolves. It won’t affect creatures that attack later in the turn.

Duergar Mine-Captain — ruling 2

If the permanent is already untapped, you can’t activate its {Q} ability. That’s because you can’t pay the “Untap this permanent” cost.

Duergar Mine-Captain — ruling 3

The “summoning sickness” rule applies to {Q}. If a creature with an {Q} ability hasn’t been under your control since your most recent turn began, you can’t activate that ability. Ignore this rule if the creature also has haste.

Duergar Mine-Captain — ruling 4

When you activate an {Q} ability, you untap the creature with that ability as a cost. The untap can’t be responded to. (The actual ability can be responded to, of course.)

Duergar Mine-Captain : règles, fonctionnement et interactions (MTG)

Duergar Mine-Captain 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

, : Attacking creatures get +1/+0 until end of turn. ( is the untap symbol.)

How it works : Duergar Mine-Captain

Original English text — Wizards of the Coast / Scryfall

#1August 1, 2008· Wizards of the Coast
This ability affects only creatures that are attacking at the time it resolves. It won’t affect creatures that attack later in the turn.
No other card is named in this ruling.
#2August 1, 2008· Wizards of the Coast
If the permanent is already untapped, you can’t activate its ability. That’s because you can’t pay the “Untap this permanent” cost.
No other card is named in this ruling.
#3August 1, 2008· Wizards of the Coast
The “summoning sickness” rule applies to . If a creature with an ability hasn’t been under your control since your most recent turn began, you can’t activate that ability. Ignore this rule if the creature also has haste.
No other card is named in this ruling.
#4August 1, 2008· Wizards of the Coast
When you activate an ability, you untap the creature with that ability as a cost. The untap can’t be responded to. (The actual ability can be responded to, of course.)
No other card is named in this ruling.