Don't worry, she don't attack Death Eaters. They're her bread; if there were no Death Eaters, her life wouldn't be as comfortable for her.
She was around during the days of Voldemort's reign, so she's used to being ordered around by Death Eaters. She don't quite let them boss her around, but she'll do missions or take orders from any of them....well, as long as it's something that puts her talents to use. Morrigan wouldn't be someone to go get your coffee. lol
Read through Ariel's bio. Sounds like they could have a relationship going either way. They'd either hate each other, or get along quite well. Morrigan isn't the subtle type, so he could easily sway her into kind of regarding him the same way she did Fenrir. She wouldn't be nearly as devoted, especially if he didn't hide the fact that he despised his father.
On the other side of it, they could have a grudging partnership. Seems Ariel is quite the opposite of Morrigan, so they could easily push each other's buttons.
By the way, Ariel's signature reminded me of a song: https://www.youtube.com/watch?v=oI80v_HELcw
Anyways, yes, I'd be happy to plot! Did you have anything particular in mind?