Font of Radiance

Epic Tier
Prerequisite: 21st level
Benefit: When you score a critical hit with a power that has the radiant keyword, the target begins to glow brightly (save ends).
    The target’s space and all squares adjacent to it are illuminated by bright light. Invisible creatures become visible while they are in affected squares, and attack rolls against creatures in those squares take no penalty for concealment. Any foe that ends its turn in an affected square (including the original target) takes 3d6 radiant damage.

Published in Player's Handbook, page(s) 207.