Question on Effect: Set AC to 10 + Dex

[Pendrake]

Hello all, I'm working on making an assassin's dagger for my party. It is a Dagger + 1, with an effect that twice per long rest it can reduce the AC of a target to 10 + dex for for one attack

The bit I'm having trouble with is I don't see a way to set AC. This: AC: 10[DEX] is doing this: +10+DexModifier.

Is there a way to set AC, not +X or -X, but AC is now X?

[Zacchaeus]

No, you can only add a modifier. So you would need to work it out manually. So if the target has an AC of 16 and it would be 13 without armour then before making the attack add a 3 into the modifier box.

Otherwise you'd need to rewrite the effect before each attack to add a negative modifier to the AC (such as AC:-3).