Creeping Tar Pit

Type: Land
Set: Doctor Who #858 (rare)

This land enters tapped.
{T}: Add {U} or {B}.
{1}{U}{B}: Until end of turn, this land becomes a 3/2 blue and black Elemental creature. It's still a land. It can't be blocked this turn.

Creeping Tar Pit has 14 printings, first in Worldwake (2010). EDHREC ranks it #3,144 among Commander cards. It appears most in Captain N'ghathrod decks (5,574 on EDHREC). 3 public decks on TCG run it. It has 2 official rulings.

Legality

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

Public decks that run Creeping Tar Pit

Masters of Evil - Doctor Who, led by Davros, Dalek Creator
Mind Flayarrrs - Commander Legends: Battle for Baldur's Gate, led by Captain N'ghathrod
Obscura Operation - New Capenna Commander, led by Kamiz, Obscura Oculus

Commanders that run Creeping Tar Pit most

Deck counts from EDHREC.

Captain N'ghathrod: 5,574 decks
Yuriko, the Tiger's Shadow: 5,088 decks
Satoru Umezawa: 4,289 decks
Gonti, Canny Acquisitor: 2,045 decks
Kamiz, Obscura Oculus: 1,284 decks
Jon Irenicus, Shattered One: 1,025 decks
Sygg, River Cutthroat: 945 decks
Umbris, Fear Manifest: 836 decks

Often played with Creeping Tar Pit

Cards that show up in the same decks on EDHREC.

Rulings

2010-03-01: When a land becomes a creature, that doesn't count as having a creature enter. The permanent was already on the battlefield; it only changed its types. Abilities that trigger whenever a creature enters won't trigger.
2010-03-01: A land that becomes a creature may be affected by "summoning sickness." You can't attack with it or use any of its {T} abilities (including its mana abilities) unless it began your most recent turn on the battlefield under your control. Note that summoning sickness cares about when that permanent came under your control, not when it became a creature.