Magic Staves

Magic staves cast Elemental and blood magic, and they are the only class that does not run on stamina. A staff spends eitr, and eitr comes from food and nowhere else.

Combat

Pros

Damage types that physical resistances do not touch. Fire has the most weaknesses of any type in the game, and lightning is the least resisted: very few enemies resist it.
Ranged attacks without ammunition. Eitr regenerates on its own, so there is nothing to carry and no bill after a fight.
It runs off a different bar entirely, so a mage keeps full stamina for dodging, sprinting and blocking while spending Eitr on damage.
Blood magic is the game's only support class. It gives summons and shields rather than damage, and because its cost is a share of your current health, it cannot kill you.

Cons

No Eitr bar means no spells. Without eitr-infused food the staff shows "Eitr required" and does nothing at all, which makes the class hostage to your cooking.
Eitr regeneration stops completely while you swim and drops to 0.8x while you block, so a sea crossing or a defensive fight leaves a mage empty.
You level separate skills, Elemental and Blood, and each one governs both the damage and the cost of its own half.
It arrives in the sixth biome, so nothing earlier in a playthrough can be solved this way.

Builds

Food is the build. Eitr comes only from eitr-infused food, so a mage gives up a food slot that a melee character would spend on health or stamina. That trade is the real decision, not which staff you carry.

Pick Elemental for damage and Blood for the team. Elemental spends pure Eitr on offence. Blood spends Eitr plus a percentage of your current health on summons and shields, which is why it is the only thing in Valheim that helps other players directly.

Regeneration rewards spending. Eitr comes back at 2 a second when the bar is full and 4 when it is empty, so a mage who holds half a bar in reserve regenerates at the slowest rate available. Being Rested doubles it outright.

Keep a shield and a one-hander on your hotbar. Block armour on the staves themselves is 48 across the class, and nothing regenerates while you swim away from a fight you could not finish.

Staves are made at the Galdr table and the Frost Foundry.

The skills are Elemental magic and Blood magic, and both level by use rather than by choice.

Best Magic Staves

Build	Name	Stats
Hardest hitter	Trollstav	500 damage across all types
Best guard	Dead Raiser	48 block armour
Most knockback	Dundr	210 knockback

Notes

Eitr starts to refill one second after the last of it is spent, at 2 points a second when the bar is nearly full and up to 4 a second when it is empty.
Eitr stops refilling while swimming. Blocking slows it to 80% of its normal speed, the Rested buff doubles it, and resting by a fire makes it four times as fast.
Blood magic costs a share of current health, not maximum health, so casting it can never kill the caster.