I came up with a custom BTD6 hero called Umbra, Hero of the Dark.
He’s a melee hero that also builds a Darkness Meter. When it fills, he automatically fires a Dark Orb that damages and pierces bloons, so he scales into real DPS later.
His main gimmick is Shadow Conversion: he has a limited resource called Shadow Energy that lets him permanently turn towers into stronger “Shadow Towers.” The tradeoff is they can never be upgraded again after that.
As he levels up, he can convert higher-tier towers (eventually Tier 5s), but higher tiers get smaller stat boosts and more special effects instead of just raw power.
He also has a Shadow Surge ability that temporarily boosts him and all Shadow Towers.
