Verna often pops with a dart and obtaining a key to her room is straightforward. From the inn entrance, w 3n will take you to a young serving boy outside a private dining room. Attempting to enter this room gets you kicked out:
'Out, riff-raff.' an extremely wealthy noble says. 'Out!'
The key is to be rich. Bring atleast 100k gold with you, and you'll be invited to sit. Do so, and a serving boy will enter. Sayto him "room key" and he'll offer you the chambermaid's key (a bar of yellow soap) for 20k. Pay him and he coughs up the key.
You can go kill Verna upstairs now for the seraphic dart, but if you want a tainted holy halberd there's one more step to take. In the southwest corner of the room is a fey rogue. Spam in and out until you catch him stealing from you. Immediately attack him, he'll stop combat and give you a Key of Mirach in return for not killing him.
Upstairs, go all south and unlock west to find Verna. She'll demand that you hand over the Key of Mirach as using it would be dangerous. So, wear and use the key. She'll become possessed, pull a halberd out from under the bed and aggro you. Kill her once, and she'll claim to be unpossessed. If you want the halberd, quickly attack her and kill her again. If not, she'll sac the halberd and re-wear her usual seraphic dart.