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One of my players uses a fighter and claims that his mark lets him use an immediate action to stop the triggering attack against an ally if the attack hits.
Does this attack stop the previous action?
(Also, an immedidate interrupt is before the trigging action, and an immediate reaction is after the trigger, right?)
A fighter's mark does not allow it to stop attacks unless he kills the target. A fighter's mark gives -2 to attacks and allows him to make a Melee Basic Attack as an immediate interrupt against an adjacent enemy who shifts or attacks a target other than him.
A fighter gets a bonus on Opportunity Attacks, regardless on if the target is marked or not.
Combat Superiority: An enemy hit by your opportunity attack stops moving, if a move provoked the attack. If the enemy still has actions remaining, it can use them to resume moving. [PHB pg 76]
Using this ability, a fighter can stop Charge attacks by effectively rooting an enemy in place before they can reach their destination. This would likely also work on any specific power that had movement before the attack as a single action, but only if the creature can not longer reach the target.
Immediate interrupts do occur before the action, while reactions happen afterwards. Note, you only get one immediate action per round.
Combat Challenge does not normally stop an attack.
Combat challenge explicitly stops movement that triggers the challenge,
From the Compendium "immediate action" entry:
If an interrupt invalidates a triggering action, That action is lost.
So if the fighter kills the target, the attack is stopped. Or if the fighter imposes some other status effect that prevents the attack the attack is stopped, but just making the combat challenge attack doesn't stop the triggering attack.
(and you are correct about the timing of interrupts and reactions)
As mentioned by other answers, a creature hit by the opportunity attack from a fighter with combat superiority will stop moving, and must take another action to move.
Special case: If a charging enemy provokes an attack of opportunity from a fighter with combat superiority, the enemy would stop moving if hit by the attack of opportunity.
To successfully complete the charge attack, the enemy would require ALL the following to be true at the time the fighter's opportunity attack hits:
If any of the aforementioned are not true, the creature could not finish the charge (and thus the attack), and the creature could take no other actions but free actions unless it spends an action point.
