Tier 1 - Slowed enemies take 25%more damage.
Tier 2 - Slowed enemy duration increased to 5 seconds
Tier 3 - Slowed enemy speed decreased to 50%
Tier 4 - Special enemies can be slowed
Tier 5 - On a slowed enemy kill, the enemy shatters spreading the slow effect to 5 nearby normal enemies (30 second cooldown)