Anyone have experience using a Night Hag?

Hey guys. I'm writing out some upcoming sessions and decided that I want to introduce a Night Hag as a minor antagonist to the party. They have previously been made aware of the hag's existence, but have not yet encountered her. The hag has been using her Nightmare Haunting ability to corrupt locals in the area and then eventually collect their souls in her Soul Bag. She then uses the corrupted spirits to construct Scarecrows which serve as her minions.

Now here's what I want to do. The group has a friendly contact that gives them some contracts. The hag has taken notice of the group and wants to manipulate them to do her bidding. So she sends her minions along with her Banderhobb servant to kidnap their friend. The night after the abduction the hag appears to one of the party members and tells them that she has their friend and will kill them if that party member does not carry out her wishes with the end goal being turning that person evil and collecting their soul.

Here's my question: How can I utilize the Hag's Nightmare Haunting ability in a manner that is will keep the game fun for the players? My issue is that the Hag can reduce a target's hitpoints each day without any resistance. Obviously she won't do so every night, but I don't think the players will think I'm being unfair whenever I use the ability on them.

So how can I keep a Night Hag fun and tense for the players?