Sorry, i've misunderstood of what you said before. I tested myself when there are only 2 units, of course its infinitely bounce until 1 unit left.
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Also what does it mean to dispel the effect? Is there a supposed spell or item that removes said effect?
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Mirror Image is the first one that i mentioned in my request, because turning something into invulnerable will instantly remove almost all the buffs from them, except buff can affect invulnerable or aura adjusted to affect invulnerable units. But i suggest to trigger the dispel effect to certain ability. Example, Unit Start effect of an ability, ability being cast equal to Mirror Image, bla bla bla, set the stack of the plague = current stack - 1 just an example, i think you'll get it
And also, Plague Siphon, its supposed to increase the caster physical damage, not make the units that affected by plague takes more physical damage. Yes, only the caster deal increased damage, i first think in my mind that the increased damage should be visible (means bonus damage, using item bonus damage), the bonus damage has 10 level (0,10,20,30,40,50,60,70,80,90),(0,100,200,300,400,500,600,700,800,900) and so on, level 1 always provide 0 bonus damage, well i don't know how to explain it, but i've done this before. it can give bonus damage (of course is visible as real bonus damage not a damage detect system tho) as % of maximum HP, current HP, mana, max mana, movement speed etc.