It'd be a marker for an enemy, and every time that enemy took damage, it would increase the combo. 0.5 for standard actions, 1.0 for skills. Or something to that effect, anyway.
I wouldn't be entirely comfortable with an ability solely designed to increase XP rewards without causing damage itself, since it'd be too open for exploitation. If you had something like:
Unstable Vortex: Energy Cost 6;
Void shifts the molecules in the air around a target charging it with his power and increasing the damage taken by them. One enemy target becomes Destabilised, receiving 20% more damage from every source until the end of their next turn.
Since this ability would trigger every time one of the others attacked the same target you'd end up gaining extra co-op combos as long as they were using abilities to damage the target. For example, Void triggers
Unstable Vortex at the beginning of the turn, Bumblebee then uses
Sting Like A Bee on the same target gaining a x2 combo multiplier triggering
Destabilised to take it to x3, Flare uses
Heat Drain pushing the multiplier to x4,
Destabilised takes it to x5 and Sigma uses
Sonic Pulse taking it to x6 and so on.
One problem I can see is if there's more than one chain going on at the same time. What I mean by that is if Heretic uses
Minor Mutation to boost the party's Attack then any skill used after that would grant both him and the other player a x2 XP bonus, but if Psy-Warp used
Telekinetic Barrier and Void then used
Void Burst on the same turn then that would also be a chain.
The simple solution is to just say that if a player triggers a skill which would allow them to link with more than one chain they only benefit from whichever is the greater multiplier, but would still count as another multiplier level for both chains.
Another problem would be skills which allow the same character to use more than one ability on the same turn, such as Heretic's
Unconfined Rage and Deimos's
Martial Prowess.
My answer to that would be that it's the number of cooperating characters which is counted as part of the chain, not the number of skills, and so in both cases they'd still only count as a single step on the multiplier ladder even if they managed to it a target twice. However they could be classed as part of two different chains if they used their skills to target different enemies on the same turn.
