Decisive Victory captures the whole essence of the Mega Man franchise. It's used to destroy a creature, then take that creature's weapon. The eight damage is for flavor purposes, as I mentioned above.
Spook is a black-style board sweeper. I needed to add some kind of lifegain effect for black, and I figure the combination of effects would be useful. As far as the actual flavor of the card, I can't say I nailed it, but it fits into black.
This falls into another niche of "standard" cards, cards that show up in a lot of sets. This is another way that green ramps their mana from time to time, and the revealing of eight cards falls into our flavor frame. That's it. That's the card.
First off, I'm pretty sure EMP doesn't need to be a rare. But by the time I realized that, I already had the whole set together, and didn't feel like reorganizing the rarities of the whole set. The effect is big, but it's really just uncommon big, not rare big. Anyways, the eight effect is here, and it's obviously a perfectly blue effect. Next.
Obviously the lifegain isn't outrageously strong; 8 life is nice (and again, the number eight), but using a card and five mana to gain that much life is kind of whatever. The damage prevention can be useful, but that's like a one-mana effect. The choice is nice enough, but this is probably the weakest of the five "8" cards.
That's the list, the "cycle" I put together. There's another cycle in the set, but for another time. Tease.