Charles, The Gatekeeper

Each consecutive spell cast by players must have a higher combined mana value than the previous one. If it doesn't, counter and exile that spell.

Until the end of your next turn, you may cast spells countered this way, using mana as though it were any color. Add to your mana pool in any combination equal to the combined mana value of exiled cards in any combination.

You don’t lose unspent mana.