You are surrounded in a flashing aura of multiple colors.
The prismatic aura spell obscures your image, offering you concealment.
In addition, any creature that hits you with a nonreach melee weapon is subject to a random prismatic effect.
Roll 1d8 and consult the table under prismatic spray (PH 264); reroll any result of 8, since a prismatic aura can't generate two effects at once.
Each time an effect is expended, that color disappears from the aura (reroll if it comes up again on the table).
Thus a prismatic aura can affect a maximum of seven attackers before it disappears, and the spell ends.
An attacker is entitled to any applicable saves, and spell resistance applies to this effect.