Heirlooms are items you equip that level with your character up to the heirloom’s cap, matching their stats to your level so that they’re always level appropriate. Heirlooms when you buy them cap out at level 60. BUT you can upgrade them so they cap at level 90, and there’s another upgrade that makes them useful until your character hits level 100.
When you buy an heirloom item it is forever added to your heirloom “collection”. The collections are shared amongst all characters on your account, regardless of server or faction. The other collections are mounts, pets, and toys. You can access the collection window through the little button at the bottom of your screen that has a horse-head icon on it.
You can pull out as many copies of an heriloom on as many characters as you want from the “master” buttons in the collection window. So no need to buy an heirloom more than once.
The upgrading is done through a consumable item that you apply to the master button in the collection window. When you use it, all copies of that heirloom on all your characters will also change to match.
Heirlooms and the upgrade consumables can be bought with:
However there are more heirloom slots that can ONLY be bought for gold and require you be a member of a guild that has completed the necessary achievements. There’s also a few others that can be found through archaeology, a level 100 shipyard mission in your garrison, or a fishing tournament in cape of stranglethorn. Blizzard will also occasionally add a special heirloom as a boss drop from the last boss of the current raid.
Heirlooms have one other primary feature: most give an xp boost until you reach their cap. So characters with heirlooms will level faster than characters without them. THIS is the primary reason why most players use heirlooms.
Heirlooms are NOT intended for your first character. The amount of currency you need would take about as long to acquire (or more) while leveling, as it would take to level a character normally. Heirlooms are intended for players who have seen the leveling content a bunch of times and are in “been there, done that” mode, BUT want another character at level cap.
Heirlooms keep their stats about on-par with dungeon blue gear of your level. At the low levels, when stats don’t exist on quest gear, this is a huge boost. After that though, it’s only because of their ever-upgrading stats that makes them “overpowered”. The game’s leveling content is tuned with the expectation that you’ll be wearing a bunch of items from the last 10-15 levels of content you’ve been through. So a weapon from 5 levels ago, pants from 8 levels ago, a cape that you can never seem to replace from 15 levels ago, etc. Heirlooms make it so any slot with an heirloom in it has stats for your current level, meaning overall you have more stats than a player in typical leveling gear.
The heirloom upgrade level 90-100 is VERY overpriced, don’t bother with it unless you have lots of gold you have no use for. The level 90 heirloom upgrade is about time and a half again the cost of the basic level 60 capped heirlooms, but it does increase their usefulness by 30 levels. However, the level 100 upgrade is twice the cost of the level 90 upgrade, and you’d need the level 90 upgrade before you can get the level 100 upgrade. That’s a lot of gold for just 10 levels of benefit.