Astral Flame

Astral flame is the burning essence of the Astral Sea. It is a fountain of fiery liquid that sheds bright light in a 20-square radius.

    Effect: It costs 1 extra square of movement to enter a square of astral flame. A creature that enters an astral flame square or starts its turn there takes 10 radiant damage per tier. A creature adjacent to an astral flame square can coat a weapon or implement in the astral flame as a minor action. Until the end of the encounter, attacks using that weapon or implement gain the radiant keyword.
    Usage: Large areas of astral flames will work well with creatures that have resistance or immunity to radiant damage, allowing them to ignore the flameā€™s radiant damage while the characters must decide if entering into the astral flame square is worth the damage.
    This terrain is also a good way to give creative characters a boost against creatures that are vulnerable to radiant damage. Use minions to guard a single astral flame square while an elite or solo leader that is vulnerable to radiant damage attacks the group in another part of the encounter area.

Published in Manual of the Planes, page(s) 21.