Okay, so like... From a
world lore perspective, it's not massively far-fetched. Yes, every magic user has the risk/potential of falling into the usage of Dhar, and there's even a particular event in the End Times where the amethyst wind of death, Shyish, gets trapped and stagnates in the mortal realm and starts "clumping" into a colossal amount of Dhar. Which makes it even easier for wizards to pick it up, and even harder for them to avoid it.
Also, technically speaking, necromancers do actually end up fighting on the side of Order at the very end... After all, there need to be living people around in order for them to turn into dead people, and then undead people... Utter annihilation doesn't suit those goals very well.
That said; given the timeline of events in the games, this entrapment most likely hasn't happened yet, so that's not really an excuse. Regardless though, getting seduced by the powerful lure of Dhar and rationalizing the usage of it against a "greater" foe sounds like something that could definitely be worked out for Sienna (even though she has a particular, specific distaste of necromancy).
...however, from a
character narrative perspective, it doesn't hold dogwater. Dwarves abhor necromancy as a desecration of the sacred ancestors. Kruber's regiment was horrifically annihilated by a necromancer and the event traumatically scarred him for life. Saltzpyre's whole career and reason for being is hunting down people who do
exactly this kind of thing.
Now, to be fair, Saltz has already sorta gone off the deep end thanks to his desperation in trying to fight the Skaven menace and the lack of official support from the order... Stuff like (
very begrugingly) working alongside a wood-witch with Kerillian's DLC career, and even going to the insane lengths of proposing a pilgrimage into the chaos wastes to seek divine aid directly... ...but even so man, a straight-up necromancer is pushing it pretty dang far.
So, yeah... I'm very curious to see what the official story ends up being for explaining how they're gonna agree to work together with Necro-Sienna. And yeah, it is kinda sad seeing my girl go off the deep end; an even deeper end than just Unchained
But ah well... Definitely looking forward to bone-rattlin' time!