After the nerf to Blackhole many people don’t bother with cleo anymore but she is still a decent hero. Many would just prefer to buy Rose or Jacques now for a raw power gain.

Arcane Codex
Through studying ancient manuscripts, Cleo has mastered arcane magic. Every time she attacks, there is a 35% chance of releasing a Dark Orb that deals an extra X damage to enemy units.
You need to have at least point in this ability as some of Cleo’s other abilities rely on it.

Book of Eclipse
After releasing 3 Dark Orbs, Cleo uses the Book of Eclipse, which causes the battlefield to be enveloped in shadow. This greatly reduces damage dealt by Humanoid Units. The eclipse lasts for X seconds.
This is a great ability to help you battle human and dwarf armies if you are playing lich, note this also effects your units so don’t use this if you are playing human or dwarf.

Tome of Pain (costs 1 mana)
The Tome of Pain contains enormously destructive dark magic. When Cleo opens it, the fury of the gods is released, dealing an extra X points of damage to enemies within a radius of 5. Additional damage is dealt when the battlefield is under an eclipse.
Not really worth the ability points Cleo’s blackhole is where it’s at you can skip this skill.

Tome of Wisdom
The Tome of Wisdom contains secrets that allows Cleo to increase the effectiveness of her troops. Whenever she reads from it, allied Mages’ Magic is greatly strengthened. Times read: X. Her Magic is increased by X.
This ability increases Cleo’s and other mage hero’s magic stat by X amount, its ok and you will likely put points into this if you are playing as human or dwarf.

Tome of Redemption
The Tome of Redemption tells how to raise the souls of the dead. With its teachings, she can revive X% of destroyed Undead Units. This is only effective on units that were destroyed during an eclipse.
This skill is useless as it is pointless to use undead units, they are simply too weak currently.

Tome of Creation (costs 5 mana)
Cleo turns the target area into a black hole, creating a large gravitational field. Enemies dragged into the black hole suffer X damage. The black hole persists for X seconds. The higher the ability level, the stronger the black hole
This skill is Blizzard on steroids, more damage and more crowd control, at the cost of more mana but does it ever destroy armies, even after the nerf it is still very powerful.

Building Cleo
Max Tome of Redemption for the massive Black Hole damage, everything else is whatever you want. Cleo mostly just used for the HUGE crowd control and Damage that is Black Hole.

Equipping Cleo
Same as Avril