Card

Second Chance

Mana cost {2}{U}

At the beginning of your upkeep, if you have 5 or less life, sacrifice this enchantment and take an extra turn after this one.

Extra-turn card: not allowed in brackets 1–2, two at most in bracket 3. Part of 60 known combos, 17 of them two-card. 4 official rulings.

Bracket

B1 Exhibition — not allowed: Extra-turn cards are not allowed
B2 Core — not allowed: Extra-turn cards are not allowed
B3 Upgraded — Not in the same deck as its early two-card combo partners: Hall of Heliod's Generosity, Muldrotha, the Gravetide, Sun Titan, Hanna, Ship's Navigator, Go-Shintai of Life's Origin, Tameshi, Reality Architect, Extravagant Replication, Court of Vantress, Conduit of Worlds, Skull of Orm, Eiganjo Dynastorian // Replenish, Silent Sentinel, Norika Yamazaki, the Poet, Harnessed Snubhorn, Dowsing Shaman, Treasury Thrull, Charnelhoard Wurm
B4 Optimized — allowed: No restriction beyond the banned list
B5 cEDH — allowed: No restriction beyond the banned list

Extra-turn card, two at most in bracket 3 (Lotus Vault house cap)
Two-card combo with Hall of Heliod's Generosity, Muldrotha, the Gravetide, Sun Titan, Hanna, Ship's Navigator, Go-Shintai of Life's Origin, Tameshi, Reality Architect, Extravagant Replication, Court of Vantress, Conduit of Worlds, Skull of Orm, Eiganjo Dynastorian // Replenish, Silent Sentinel, Norika Yamazaki, the Poet, Harnessed Snubhorn, Dowsing Shaman, Treasury Thrull, Charnelhoard Wurm (early with Hall of Heliod's Generosity, Muldrotha, the Gravetide, Sun Titan, Hanna, Ship's Navigator, Go-Shintai of Life's Origin, Tameshi, Reality Architect, Extravagant Replication, Court of Vantress, Conduit of Worlds, Skull of Orm, Eiganjo Dynastorian // Replenish, Silent Sentinel, Norika Yamazaki, the Poet, Harnessed Snubhorn, Dowsing Shaman, Treasury Thrull, Charnelhoard Wurm)

Bracket: Lotus Vault analysis, with Commander Spellbook flags

60 known combos, 17 of them two-card.

2022-12-08 · Wizards of the Coast
If multiple "extra turn" effects resolve in the same turn, take them in the reverse of the order that the effects resolved. In other words, the most recently created extra turn is taken first.
2010-06-15 · Wizards of the Coast
On resolution of the trigger you have to sacrifice this card if you have 5 life or less. If you can't (because it is not on the battlefield, for example), then you still take an extra turn after this one.
2004-10-04 · Wizards of the Coast
It is not optional.
2004-10-04 · Wizards of the Coast
The ability will only trigger if you have 5 or less life at the beginning of upkeep. This is checked again at the start of resolution and nothing happens if you do not still have 5 or less life.

Rulings: Scryfall · Wizards of the Coast