Let’s analyze Spell Queller’s triggered abilities:
When Spell Queller enters the battlefield, exile target spell with converted mana cost 4 or less.
When Spell Queller leaves the battlefield, the exiled card’s owner may cast that card without paying its mana cost.
We can imagine immediately that the two abilities of Spell Queller are harbingers of many interesting interactions. Let’s examine the most peculiars.
Countering the Uncounterable
The spirit’s enter the battlefield ability exiles spells. This makes it able to interact with those spells that “can’t be countered”. A spell that can’t be countered, however, can be exiled. Thus, Spell Queller, will be able to exile spells like Supreme Verdict, Abrupt Decay or Thrun, the Last Troll.
Past in Queller
Spell Queller’s ability exiles spells with converted mana cost 4 or less. The converted mana cost of an object is a number equal to the total amount of mana in its mana cost, regardless of color.
For example, we can legally target a Past in Flames casted for its Flashback cost: the alternative casting cost, in this case Flashback, doesn’t change the converted mana cost of an object. But what happens to the “Flashbacked” Past in Flames if we try to exile it with the spirit’s ability?
A spell casted for its Flashback cost goes to exile, instead of any other zone, when it leaves the stack. This replacement effect, however, does not interfere with Spell Queller’s ability that will move the object right in exile. Once the spirit will leave the battlefield, and its second triggered ability will try to resolve, the Past in Flames may be launched.
The converted mana cost of a spell with X in its cost is treated as the number chosen for it while the object is on the stack. If, for example, our opponent casts a Mind Twist with X=3, the converted mana cost will be 4. The spirit will be able to easily banish the upcoming headache.
When the spirit’s “leave the battlefield” ability will allow our opponent to cast Mind Twist without paying its mana cost, the only legal choice for X will be 0… definitely better than an aspirin.
Can a spirit handle forever the Wrath of God? Well, yes. Let’s say we respond to a spell with my Spell Queller. In response to its first triggered ability, retaining priority, we bounce the spirit with Unsubstantiate. When the spell resolves, Spell Queller’s “leave the battlefield” ability will go on the stack and will resolve before its ETB ability, doing nothing useful. When the first ability will resolve, the spell will be exiled forever.
If this reminds you an analogous interaction possible with Oblivion Ring and similar abilities, you have a good memory.
Andrew casts Collected Company. Nicole responds with Spell Queller, it resolves. With the triggered ability still on the stack (that targets Collected Company), Nicole exiles the spirit with Displacer’s activated ability.
During the resolution of Displacer’s ability, both the abilities of Spell Queller will trigger. Nicole will choose the order they’ll go on the stack. Even if we first let resolve the ability that lets our opponent cast the spell (as from previous scenario), we will still have a new ability that will try again to exile the Collected Company, seen that Spell Queller’s ETB ability isn’t optional and CoCo is the only legal target in this scenario. Not the best way to tap our creatures and use our mana.
Can we exile again the already exiled spell?
Unfortunately, we can’t do it.
Once the angel’s ability resolves, both Spell Queller’s abilities will go on the stack in the order we prefer. Even if we choose to resolve for first its “leaves the battlefield” triggered ability, the exiled spell will not be a legal target for its “enters the battlefield” one, because ability’s targets have to be chosen as soon as abilities goes on the stack, and not when they resolve.
During the resolution of Spell Queller’s second ability, we can cast the exiled card without paying its mana cost. This prevents us from paying for the spell with alternative costs, but we can still choose to pay any additional cost. For example: we can pay the additional cost of a Borrowed Grace in order to use both of its modes. Sometimes, additional costs aren’t optional, like in the case of Bone Splinters, even if we don’t have to pay for its mana cost we’ll still have to sacrifice a creature. Is to be noted that additional costs do not change a spell’s converted mana cost. A Kicked Burst Lightning is still a legal target for Spell Queller’s ability: Even if we’ve spent 5 mana to cast it, its converted mana cost will still be 1.
As we have said, only a single alternative cost is allowed. If we try to play a Den Protector that has been exiled with the spirit’s ability, we will not be able to cast it with Megamorph, but only as a humble warrior.