Shipbreaker Kraken

Mana cost: {4}{U}{U}
Type: Creature — Kraken
Power / toughness: 6 / 6
Set: Theros Promos #63★ (rare)

{6}{U}{U}: Monstrosity 4. (If this creature isn't monstrous, put four +1/+1 counters on it and it becomes monstrous.)
When this creature becomes monstrous, tap up to four target creatures. Those creatures don't untap during their controllers' untap steps for as long as you control this creature.

Shipbreaker Kraken has 5 printings, first in Theros (2013). EDHREC ranks it #8,972 among Commander cards. It appears most in Kiora, Sovereign of the Deep decks (3,123 on EDHREC). 1 public deck on TCG runs it. It has 7 official rulings.

Legality

commander: legal
legacy: legal
vintage: legal
pauper: not legal
modern: legal
pioneer: legal
standard: not legal
brawl: legal

Public decks that run Shipbreaker Kraken

Reap the Tides - Commander Legends, led by Aesi, Tyrant of Gyre Strait

Commanders that run Shipbreaker Kraken most

Deck counts from EDHREC.

Kiora, Sovereign of the Deep: 3,123 decks
Runo Stromkirk // Krothuss, Lord of the Deep: 2,744 decks
Aesi, Tyrant of Gyre Strait: 1,900 decks
Arixmethes, Slumbering Isle: 1,764 decks
Kenessos, Priest of Thassa: 1,454 decks
Charix, the Raging Isle: 276 decks
Slinn Voda, the Rising Deep: 128 decks
Tromokratis: 117 decks

Often played with Shipbreaker Kraken

Cards that show up in the same decks on EDHREC.

Rulings

2013-09-15: An ability that triggers when a creature becomes monstrous won't trigger if that creature isn't on the battlefield when its monstrosity ability resolves.
2013-09-15: If you lose control of Shipbreaker Kraken between when its last ability triggers and when that ability resolves, the target creatures will become tapped. However, they won't be affected by the other effect and will untap as normal during their controllers' untap steps.
2013-09-15: Monstrous isn't an ability that a creature has. It's just something true about that creature. If the creature stops being a creature or loses its abilities, it will continue to be monstrous.
2013-09-15: If another player gains control of Shipbreaker Kraken, the creatures will no longer be affected by the effect preventing them from untapping, even if you later regain control of Shipbreaker Kraken.
2013-09-15: If you gain control of a creature that's being prevented from untapping, that creature won't untap during your untap step for as long as you control Shipbreaker Kraken.
2013-09-15: The ability can target a tapped creature. If a targeted creature is already tapped when it resolves, that creature just remains tapped and doesn't untap during its controller's untap step.
2013-09-15: Once a creature becomes monstrous, it can't become monstrous again. If the creature is already monstrous when the monstrosity ability resolves, nothing happens.