Riot Rewarded me for Good Behavior...

Think of it this way:

• Veigar's Goal is to deal damage (duh).
• You do that by getting AP and getting levels.
• Most games, you won't get to full build
  • So the question is, how can you get the most AP and levels?

Veigar has two ways he gains AP with his passive:

• If he lasthits a minion or monster with his Q, he gains 1 AP.
  • 2 AP if it's a cannon minion or large monster
• If he kills an enemy champion with any source of damage, he gets 1 AP * level of Q (1 AP at Q lvl 1, 5 AP at Q lvl 5).

If Veigar focuses on farming his Q on minions, he can get, at max, 8 AP per wave (if it has a cannon minion). This costs a whopping 490 mana (lvl 3 Q)! That's pretty much his whole mana pool at lvl 5 (when he has q lvl 3).

If he uses those same 7 Q's on his opponent, he's just dealt 1,190 (+ 420% AP) damage to them.

If Veigar focuses on csing with his Q, he doesn't have a way to trade with his opponent. That means his opponent is going to be harassing him constantly, which means he's likely going to miss CS. It also means he'll be OOM, which means his opponent can easily shove the wave and force him to cs under tower. Csing under tower with veigar, when you're oom.... is not a fun time.

Since you're getting harassed and have no mana, you're going to need to go back more often. That means you'll be missing out on XP, and levels matter a LOT to veigar, because he relies on his stun, which scales well with levels, to lock his opponent down long enough to hit his W.

So basically, if your opponent is bad and is just letting you farm your Q without punishing you for it, sure, go ahead. But any time I see veigar use his Q on a minion, I go stand between him and his minion wave for the next 4 seconds, because I know I'll win any trade. Okay, that's a slight exaggeration, but Veigar isn't getting any cs during those 4 seconds, or he's losing a lot of hp to do so.

The 1 AP you get from stacking your Q is worth around 22 gold. A minion is worth around 22 gold. So if you miss 1 CS because you used your Q on a minion, you just broke even. If you miss 2 cs, you lost out. If you miss 1 cs but you took a lot of harass or lost a trade, you lost out. This is why you shouldn't cs with Q unless:

1. You're going back soon and you can afford to get low or run yourself oom.
2. You have teleport and you can afford to get low or run yourself oom.
3. Your opponent is dumb and you can afford to get low or run yourself oom.
4. Your team can stall until you reach full build, at which point you can't get more AP from items but you CAN get more AP from Q stacking.

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So basically stacking his Q on minions is just a noob trap, unless it's a cannon mini/super mini, OR you're about to back or something and can afford to waste the mana real quick?

The passive gain for any source is only on champions, right? Basically meaning that stacking his Q shouldn't be a huge priority, so much as a secondary objective, unlike Nasus?

Stacking his Q is his main priority until laning phase ends. It doesn't seem like much, but if all goes well in a game, you should have over 100 extra AP at 20 minutes. If it still doesn't seem like much, will enhance it, and you'll have around 150 ap plus whatever items you have at the time. If you have this much ap at 20, nothing can stop you as long as you don't do anything stupid.