You should definitly get a Hextech Gunblade over Will of the Ancients(WotA). It gives you more damage (your abilities scale of Attack Damage as well) and it has an active CC spell, and Katarin lacks CC. I would definitly always build Hextech Gunblade as first item on her, and I saw many pro players (like Scarra, who is known for his Katarina) do the same, simply cause you need every single stat of the item.
You used to get WotA on Katarina in double AP team compositions, but since they nerfed it pretty hard, its not that common anymore, and I wouldn't buy WotA.
UPDATE 2016-06-23
Champion Shards can now be used to upgrade champion mastery levels.
To get mastery tokens for level 6 you need to earn an S- at the end of a game and to earn mastery tokens for level 7 you need to earn an S rank at the end of a game with a level 6 champion mastery.
To unlock level 6 combine 2 Mastery tokens with a champ shard of the same champ, OR the champ permanent of the same champ, OR 500 blue essence.
To unlock level 7 combine 3 Mastery tokens with a champ shard of the same champ, OR the champ permanent of the same champ, OR 600 blue essence.
How do you earn free loot?
You can earn free chests or earn free keys. To win free chests you, or some one you have queued up with must obtain a S-, S, or S+ rank. Only 4 chests can be obtained within a month and only one chest per S rank per champion per season. (Important to note if someone you queued up with gets an S rank and you earn a D rank on lets say graves, graves will count towards the only 1 per season).
To obtain keys, you must win a game and after winning you have a chance to obtain a key fragment. 3 key fragments combined equal one key. Then use the free key to unlock a chest.
What's the deal with essences and why are they so important?
There are two types of essences champion and cosmetic. Champion essence comes from disenchanting champion shards, which then can be used to unlock a champion permanently.
Cosmetic shards are for skins, champion and ward. You can earn more cosmetic shards by disenchanting skins. You can then use the cosmetic shards to unlock a skin permanently.
What do the different options mean when dealing with Redeeming Re-rolling Disenchanting and Upgrading?
Redeeming - lets you use the champion or skin for 7 days.
Re-rolling - requires three shards of the same type, guaranteed to give you a champion or skin that you do not already own.
Disenchanting - gives you more essences of that type.
Upgrading - use essences to make that shard a permanent unlock.
That should cover how it all works.
Best Answer
That symbol indicates a legacy skin (a skin that you can no longer buy in the shop). Source