Duergar Mine-Captain

• WWizards of the Coast2008-08-01
  Official Ruling
  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.
• WWizards of the Coast2008-08-01
  Official Ruling
  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.)
• WWizards of the Coast2008-08-01
  Official Ruling
  If the permanent is already untapped, you can’t activate its ability. That’s because you can’t pay the “Untap this permanent” cost.
• WWizards of the Coast2008-08-01
  Official Ruling
  This ability affects only creatures that are attacking at the time it resolves. It won’t affect creatures that attack later in the turn.

Rulings via Scryfall