"There will be a sacred area underneath the tower that kills a challenge.
Towers at the sacred location may wear Sacred Robes and serve as an Acolyte."
Sacred Robes: Unique item
Pacifist in training: deals -90% damage
Favor: TBD beneficial effect for all towers.
Darkness: Hel's favor: -10% base cooldown
Nature: Freya's favor: +50% speed
Metro: Headmaster's favor: +50% xp
Light: Tyr's favor: +10% base damage
Or favor could be universal (not based on the tower element).
Taunt: -10% creep speed
Expose weakness: creeps take +10% damage
The idea is that taking a strong tower out of the game in exchange for a global buff could be worth it.
Or the prophecy could be used by making a secondary tower strong enough to deal the killing blow to a challenge and the global buff is won for minimal cost.
If explaining the element specific effects is too complicated, then it could be a random effect or one effect could be the one selected for the Robes.
Edit: this isn't balanced the way I'd imagined. I was imagining a unique tower as the carry and then only that tower would be able to wear the robes. If Solara killed the challenge and any Solara can wear the robes, then that isn't a penalty.
Andy, can the robe be made to apply to a certain grid location? Maybe the lore could be changed to the ground beneath the killing tower is marked and that's the only location where the effect is active. Otherwise the robes are effective as a handbag on Holgar without Ila's elixir.
Attempt to rebalance the card and revise to align with Andy's prophecy wording.