Feel free to suggest more and suggest edits for these!
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1.) How Did We Get Here?: You're here, aren't you? Participate in a game of Dawn of the Dead (everyone gains two health potions)

2.) Double Kill: Kill two zombies in a single turn (grants 5 additional HP)

3.) Triple Kill: Kill three zombies in a single turn (grants 7 additional HP)

4.) Overkill: Kill five or more enemies in a single turn (regain 1 spell slot)

5.) Decimation: Kill 10 or more enemies in a single turn (adds 2d8 to any weapon damage)

6.) Flamin' Hot: Kill several enemies using a fire-related weapon or spell (grants 10 additional HP)

7.) Stay Frosty: Kill an enemy using an ice related attack

8.) That Was Close!: Finish your first wave (gain an additional 15 HP)

9.) No One Said There Would be More...: Finish 5 waves (add 3d8 damage to any weapon you have and gain 2 health potions)

10.) So. Many. Zombies.: Survive 10 waves (regain all hit points immediately, gain 3 health potions)

11.) Juggernaut: Become a Super Zombie (gain a damage resistance of your choosing)

12.) Not So Invincible: Kill a Super Zombie (the entire party regains all hit points)

13.) In the Eye of the Beholder: Kill the zombie Beholder (entire party regains all HP and spell slots immediately)

14.) Field Medic: Revive a downed party member (gain 2 health potions)

15.) I Got You!: Heal a team member while in combat (regain one spell slot)

16.) Looter: Find your first weapon or armor in the mansion (grants 5 additional HP)

17.) Don't Mind Me...: Find your first potion in a cabinet, chest or drawer

18.) Ooo, Shiny!: Find a weapon or armor of legendary quality (add 1d12 extra damage to a weapon)