Keeps unspent mana
Unspent mana isn't lost as steps and phases end, or turns into another color instead of being lost: Omnath, Locus of Mana; Upwelling; Horizon Stone; Kruphix; Ozai, the Phoenix King. A mana burst that keeps only "this mana" until end of turn (Neheb, Dreadhorde Champion) is not this, nor is making a player lose unspent mana (Mana Short).
Cards that do this
- Electro, Assaulting BatteryLegendary Creature — Human Villain
- Ashling, Flame DancerLegendary Creature — Elemental Shaman
- The Last Agni KaiInstant
- Leyline TyrantCreature — Dragon
- Ozai, the Phoenix KingLegendary Creature — Human Noble
- Omnath, Locus of ManaLegendary Creature — Elemental
- Horizon StoneArtifact
- Kruphix, God of HorizonsLegendary Enchantment Creature — God
- Omnath, Locus of AllLegendary Creature — Phyrexian Elemental
- Fangorn, Tree ShepherdLegendary Creature — Treefolk
- Omnath, Locus of the VoidLegendary Creature — Elemental
- UpwellingEnchantment