Dannamoth,I've extensively tested Season 3 on PBE already, and i think it's safe to say that he will have an even harder time now:

-Base MR and tankiness runes remain unchanged, while every single magic pen item & rune is nerfed. For reference, that Sorc + Guise build you mentioned goes from -50MR to -37.8MR, and the change in penetration order won't make up for it at all (not before ADC takes over the game at least).

-AP items are weaker (just look at Deathcap) which makes burst mages like Xe less likely to kill someone, with the side effect of also making the AP->Armor passive weaker.

I think in that context it would be justified to buff him a little, similarly to how junglers like Sejuani, Rammus and Hecarim got adjustments for the S3 jungle. Do you think that would be possible?

I'd have to say that Athene's is pretty much a core item, because when you can poke for minutes on end without worrying about mana you have great long-term power. This item gives him his seigeing strength, because early on it chunks people without caring about MR, and in stand-offs or the occasional poke it becomes a nuisance for the enemy team. I love being able to pot-shot people for 300-400 damage without retaliation, Champions without significant health or regen just can't take that kind of abuse.

With Season 3 he will become immensely stronger, once again doing true damage to anyone with less than 105 MR with a full magic pen build. His weaknesses are assassins and health stacking throughout the game, just because pure burst is naturally countered by it.

As for the whole AP Kog vs Xerath: They have different ways in how their seige is better than the other. Kog Ult has better range and can be used on the go but is better for single Target nuking and can reach insane levels of Mana Cost. Xerath can hit more enemies with his Q and has the same damage as Kog Ult when late game AP is considered with better Magic potential magic pen, but relies on AP instead of Base Damage and has to root himself to gain the range and penetration. Also realize that Kog's only defense against a charging enemy is a Slow (albeit in AoE), while Xerath can stun and has armor to help defend.

Why do people build void staff on Xerath? Won't it only stack multiplicatively with the %pen in Xerath's kit? Am I missing something?

They do that because you want to be rooted like a giant "KILL ME" sign as least as possible.

Void Staff gives you basically the same thing, without needing to immobilize yourself with a 5 skillpoints ability that arguably helps enemies as much as yourself. If you do happen to use it, then Void Staff makes it better anyways.

wait, the passive is % reduction not penetration? I thought it was penetration...

So you're saying it's for people who don't like to use Xerath's kit? Also, the choice to not build void staff doesn't mean you're neglecting your damage - AP actually gives kickbacks to armor from your passive, and flat pen will synergize much better with your kit %pen come S3 (even at level 1 in that spell). Seems to me that void staff is a poor choice on Xerath.

By far the champ I fear on the enemy team the most as Xerath is Nocturne. It's very easy for him to nullify a lot of your damage by blinding you when you're about to burst, and has one of the best gap closers against you, not to mention his shield will disjoint any attempts to use stun to get away.

Nocturne and Sivir are really difficult to deal with because of the spell shields. Morgana can be an annoying lane to play against as well because she pushes just as well, if not harder than Xerath does.

Xerath's kit is only good for one thing, bro: damage. Xerath has no utility except a single-target stun that must be performed in 2 parts and one of those is a skillshot, that's already pretty ridiculous for an AP carry. Did you realize that Draven and Varus (AD carries, oh god) sit on Xerath's face in terms of CC?

Xerath can't peel.

Xerath's effects are delayed on every single one of his spells, except arguably Mage Chains (E) at melee range. In the whole Flash+Ignite+Gapcloser+Omgwtfbbq meta, that's another ridiculous disadvantage. Said delayed effects also do absolutely nothing if you die before they land (and everybody knows it and abuses it). It's like if Ezreal died to an Ignite after firing his ult, ult would deal no damage. Or teemo shrooms doing nothing while he's dead. In actual play it's just a disgusting thing to witness.

Xerath doesn't deal damage based on %missing HP or current HP or max HP.

So when you're talking about his kit, no wonder people wouldn't want to use said kit, it's bad. It's a fact, we have to deal with it. Compare with Blitz who would certainly still be useful with any randomly-generated set of items.

Ultimately, Xerath can't do anything useful for his team other than damaging. He thus needs every single bit of AP and Mpen he can get, because his ratios are good at the very least.

Not getting Void Staff makes him completely useless against high MR targets, unless he roots himself. That's precisely when those same targets who unfortunately happen to be CC tanks or bruisers most of the time, jump at him and tear him a new one, and he can't do anything about it.

If you've played Xerath in ranked feel free to share your experience too, because i have, and that's what i've learned from it.

I think Xerath's damage is weaker compared to most mages but I find this as a way to balance him. Like most burst he is squishy and although his damage isnt as high he exceedes almost if not all other champ ranges giving him an advantage if used correctly. If he had the damage of a Katarina combined with his range he would need a nerf badly.