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The Restless Dead

Conjuration
Dark
Enchantment
Level 4
Charges: 3
Whenever a living enemy dies, raise it as a skeleton.
Raised skeletons have max HP equal to that of the slain unit, and deal 5 physical damage in melee.
Skeletons of flying units can fly.
This effect lasts 20 turns.

Attributes:

20Duration
5Minion Damage

Upgrades:

2 - Junk Golems
Whenever an enemy constructs dies, raise it as a junk golem.
3 - Elemental Spirits
Whenever an enemy fire, ice or lightning unit dies, a random summoned ally gains 100 resistance to that element and a ranged attack of that type. Each ally can gain only 1 such buff.
2 - Restless Minions
Allies can also be raised by Restless Dead