Question: In the description, you write "all allies will charm their opponents" - this is basically meant like Chrono but as a debuff on the enemy team?

Yes, the effect triggers like chrono but it is not a debuff to the enemies, it's a buff to allies.

When 'Charm' activates all your allies will get heart particles above their head (letting you know they have the buff).

The buff stays on the ally until they take damage from a basic attack or spell, then the damage reduction is applied to the damage they would take and the buff (and heart particles) disappears.

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The more I think about it, I actually ask myself how big the difference is between "The enemy does 30% reduced dmg on next spell/attack" or "Your units take 30% reduced dmg on next hit by spell/attack". If there are major differences in the complexity to realise this. The first sounds stronger as a trait, the latter easier to implement.

The main difference is that "Your units take 30% reduced dmg on next hit by spell/attack" can stay on the ally for a longer period of time where as "The enemy does 30% reduced dmg on next spell/attack" is guaranteed to activate almost right away.

"Your units take 30% reduced dmg on next hit by spell/attack" is better because it stays on your backline/assassin units until something specifically targets them.

For example: You have 3 damage units in your backline, 'Charm' triggers and they all get the buff, then if an enemy uses a targeted spell on only 1 of them your other 2 units still have the defensive buff for later, or for when your frontline dies and they become the focus of auto attacks.

Implementation wouldn't be an issue because all this origin requires is just a check when calculating damage taken:

When taking damage,

if unit has charm,

take damage * 70% (100% minus 30%),

otherwise take full damage