Q: My opponent uses a spell or ability that can't target Spellskite. Can I change its target to Spellskite to cause it to fizzle?A: No. You can only change a target to something that is itself a legal target. If the spell or ability you're targeting couldn't have targeted
Spellskite 
normally, you can't force it to do so for the same reason your opponent couldn't target it in the first place.
Note that you
can use Spellskite's ability--it just won't accomplish anything since what it's trying to do is impossible.
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Q: So how can I use Spellskite to fizzle my opponent's spells or abilities?A: If your opponent casts a spell or uses an ability that can target Spellskite, you can change the target to Spellskite
first, and then
after that give Spellskite shroud somehow (or do something else that would make it an illegal target). The target has already been changed to Spellskite, so the fact that it has suddenly become an illegal target won't change it back. The spell or ability will try to resolve, but if all its targets are illegal it will fizzle instead.
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Q: Does Spellskite work against the Equip ability of my opponent's Equipment?A: No. The Equip ability can only target a "creature you control", and your opponent doesn't control Spellskite, so it's an illegal target for the Equip ability. (As before, you can use Spellskite's ability if you wish--it just won't accomplish anything.)
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Q: Does Spellskite work against Auras?A: If an Aura spell could legally target Spellskite, yes, you may use Spellskite's ability to change its target. If not, no. (So yes to
Holy Strength 
, no to
Evil Presence 
.)
Note that an Aura that's already on the battlefield is not a spell and is not targeting anything, so you can't use Spellskite's ability to move it.
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Q: If a spell or ability has multiple targets, can I change more than one of them to Spellskite?A: Maybe, but most likely not. A spell or ability can only target the same object once for each instance of the word "target" in its text--so for example, something like
Repel the Darkness 
can't be cast targeting the same creature twice, and Spellskite can't force it to do so either. (You also can't change the number of targets.) And even if the spell uses the word "target" multiple times, if it specifies the targets need to be different you can't change more than one of them to Spellskite.
Arc Trail 
works this way, for example: it specifies that the second target must be "another" creature or player, ie one that's different from the first.
However, there are a few rare spells that do neither of these things--they use the word "target" multiple times and don't say the targets need to be different. Cards like these
can have multiple targets changed by Spellskite.
Seeds of Strength 
is an example of such a card.
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