Custom Magic Weapon: Whisper

[WarrentheHero]

I'm a DM and I've introduced a recurring Assassin against the party. In addition to being a high-level Assassin, her main strength is she has a Magic Dagger called Whisper, and I want to run the stats by you guys to see if you think they're too strong/weak to use against a party of levels 6-7 (and increasing). Eventually the players will get the dagger for themselves.

Whisper
Very Rare, Dagger
Requires Attunement
This magical dagger rewuires attunement and grants you +1 to attack and damage rolls. Additionally, it has 3 charges, and regains one charge each day at dusk. You may spend one charge to cast Invisibility on yourself, following the rules for he spell as normal. Additionally, you may spend one charge to imbue Whisper with power. Within the next minute, your first hit with the dagger places the target under the effects of an isolated Silence spell. This use of Silence does not use your Concentration.

The main attack pattern is that she starts invisible or hidden, Sneak Attacks a spellcaster and silences them, and then continues her attack with a bunch of thugs. If things go bad, she uses Invisibility to run away.
I'm concerned that the auto-Silence might be a bit too strong. My players seem to like her as a recurring threat, but the Arcane-Domain Cleric gets frustrated when Silenced because he can't do anything afterward, and she usually targets him.

[Ziegander]

I would say that the silence effect should read as something more like this:

"Within the next minute you and Whisper are effected by a Silence effect as the spell. During that time, if you hit another creature with an attack made with Whisper you may transfer the Silence effect onto the struck creature as a bonus action. The struck creature is entitled to a Charisma saving throw to negate the effect (which negates the effect on you and Whisper as well), and if their initial save fails they may make a new save at the end of each of their turns to remove the effect. After being transferred to a new creature, the Silence effect lasts until the initial minute is up."

Not sure what the save DC of the effect should be, possibly 8 + your proficiency bonus + your Dexterity bonus?