Ah, Kamigawa block. Among the most detested of all time, despite its rich lore and complex draft format which makes the highest-level players revere it for its difficulty. So many things went wrong there. One of them, given its storm scale rating of 9 (meaning it is highly unlikely to ever return), is Splice.

Mechanics of Magic: Splice

I'm thinking... Electrical Wires

Let's start with the actual rule, shall we?

"Splice is a static ability that functions while a card is in your hand. 'Splice onto [quality] [cost]' means 'You may reveal this card from your hand as you cast a [quality] spell. If you do, that spell gains the text of this card's rules text and you pay [cost] as an additional cost to cast this spell.' Paying a card's splice cost follows the rules for paying additional costs in rules 601.2b and 601.2f-h." (Comp. rules 702.47a)

One joke which runs through the Magic community quite a lot is every mechanic is basically a variation of Kicker. I don't actually agree, but in this case, it kind of is. The gist is this:

You cast a spell. You pay an additional cost, and your spell gains an additional ability. That sounds like Kicker.

The big difference is instead of the extra ability being on the card you're casting, this is a spell in your Hand. So, if you're casting a spell with the subtype "Arcane" (more on this in a minute) like, say, Choice of Damnations (you monster!), you may reveal Strange Inversion from your Hand as you do. If you do, you pay two and Strange Inversion adds its effect onto Choice of Damnations, so in addition to making someone pick a really difficult number, you also get to make their 1/5 Ox a 5/1 (for some reason).

I'm thinking... Genes

There are so many weird things about this.

First, there are 30 total cards with Splice, as of this reading, and all but three of them are "Splice onto Arcane." That's why I mentioned Subtypes earlier - originally, the ability was designed to work only with Subtypes (Arcane, Eldrazi, that sort of thing), but a group of designers really wanted to do "Splice onto Instant or Sorcery." Instants and Sorceries are Types, not Subtypes, so they had to change the rule, which is why the rule says "[quality]" and not "[subtype]". Most likely, if you encounter this ability, it'll be "Splice onto Arcane," which means in order to Splice the spell, you'll need to have an Arcane spell to cast.

Then there's the weirdness around what happens when the spell Splices.

Let's say you're Splicing Wear Away onto Lava Spike. You cast Lava Spike, reveal Wear Away, pay the cost, then choose to target their Oblivion Ring or whatever. Because you've Spliced Wear Away onto Lava Spike, Lava Spike is the card destroying the Enchantment. That means the spell is Red, so any effect giving the Oblivion Ring protection from Green won't protect it from this particular version of Wear Away. Basically, the spell with Splice loses its own qualities (color, type, &c.) and gains the qualities of the spell onto which it is Spliced. If you Splice an Instant onto a Sorcery, the Spliced spell becomes a Sorcery!

Several more things here.

• You can't Splice a spell if the Spliced spell doesn't have legal targets. (In the previous example, you can't Splice Wear Away if there are no Artifacts or Enchantments to target.)
• You can only Splice a card once per turn onto a single spell. If you Splice Goryo's Vengeance onto something from your Hand, you can't reveal it again and Splice it again to do it twice. You have to cast another Arcane spell and Splice it onto that.
• It's worth pointing out you can splice Goryo's Vengeance onto another Arcane card you cast this turn, though. If you have two Lava Spikes and want to Splice the Vengeance to both of them this turn, go for it.

• Note if you have multiple copies of the same card with Splice in your Hand, you may Splice them both. So if you have two copies of Goryo's Vengeance, you may Spice them both onto the same spell.
• You can Splice more than one card onto a single card, though. So you can Splice Wear Away and Goryo's Vengeance onto that Lava Spike on the same casting, assuming you have all legal targets.
• If you do this, you reveal them all at once, then choose the order in which they resolve. You must reveal all the Splice cards at the same time.
• You choose targets normally. The spell doesn't resolve if any of the targets become illegal. Shifting Borders would fizzle if either of the two Lands didn't exist as it attempts to resolve.
• Once the primary spell (with its Spliced additions) leaves the stack, because of resolving, being countered, or any other reason, it loses all additional abilities the Splice added. So that Lava Spike is just a Lava Spike once it's done.
• You can't Splice a card onto itself, because the card is on the Stack, not in your Hand, when you attempt to Splice it.
• If the Spliced card's text targets something, and the original card also targets something, they can target the same or different things.
• On the other hand, if you splice a targeted card onto an untargeted card (like Splicing Kodama's Might onto Path of Anger's Flame), if the target for Kodama's Might is illegal by the time it attempts to resolve, the entire spell won't resolve because of the illegal target.

I'm thinking... Apple Trees

There are a couple of cards worth pointing out.

There are three spells with Splice onto Instant or Sorcery.

Then there's this little gem.

This spell can only be Spliced (or cheated into play via something like Cascade). You can't just pay mana and cast this spell.

Finally, because Splice adds the effect of the Splice card to the card being cast (normally the Arcane spell), if the main spell gets counters, the Splice's effects will be countered as well.

Thanks for reading.