Umbral Souls grant varying Buffs depending on the kind of Skeletal Minion being replaced, as follows -

Umbral Souls from:
• Skeletal Warriors grant 15% increased Attack Damage.
• Skeletal Snipers grant 15% increased Projectile Speed.
• Skeletal Clerics grant 30% increased Energy Shield Recharge Rate.
• Skeletal Arsonists grant 15% increased Area of Effect.
• Skeletal Storm Mages grant 25% increased Spell Damage.
• Skeletal Frost Mages grant 35% increased maximum Energy Shield.
• Skeletal Brutes grant 60% increased Stun buildup.
• Skeletal Reavers grant 6% increased Skill Speed.