Arcane Disruption (4e Trap)
From D&D Wiki
Magical Runes are drawn onto the floor. Seeing it with perception only tells the players this. The players must make an Knowlege: Arcana check, DC 20, to identify this obscure Hazard and it's effects, (tell them that it may cause their own arcane spells to be cast on them if they succeed the knowlage check) If they Score a 10, say that the character realises it says something about Disruption and spells and if they score a 15 then they understand that it seems to disrupt spells in some way that they do not fully understand.
When a being tries to use an arcane power while touching or standing in a square with an Arcane Disruption rune on it, the caster struggles with the hazard as it attacks their willpower and if they fail, the spell seems to backfire and is cast on them. It must still overcome their defenses in order to effect them.
This is best used to add an extra challenge to encounters, as any arcane casters must take a risk every time they want to attack. Double the XP of Encounters that take place in areas with Arcane Disruption due to the aditional challenge.
To level it up for higher level adventures, increase the attack bonus by at least 1 for every level (+6 vs. Will at level 2, +9 vs. Will at level 5 and so on).