Tessus, the River Keeper

Instant, sorcery and wizard spells cost {u/r} less to cast. This effect reduces only the amount of colored mana you pay. (For example, if you cast a spell with mana cost {1}{u}{r}, it costs {1}{u} or {1}{r} to cast.)
Although he can control the fury of the river at will, some would argue it is better to rely on the river's unpredictability.