Rainbow Snake

Haste, Vigilance, Trample, Reach
Whenever Rainbow Snake attacks or blocks,
add {w}{u}{b}{r}{g} to target opponent's mana
pool. This mana must be used to generate a
spell by the end of the turn, otherwise it
deals damage to that player equal to the
amount of mana that wasn't converted into
a spell. Spells your opponents cast after
this effect have flash.