Vindasel is an Ayleid Ruin where Umbra took asylum after many years of challenging people to fights. She was obsessed with her sword Umbra that is able to capture souls, and always hungers for them. You will need to go to Vindasel to defeat her and although it makes things easier if you are a higher level, you do NOT need to complete the Clavicus Vile quest. If you simply go to Vindasel, and are a high enough level to defeat Umbra. Make sure that you have a high restoration level (at least journeyman). Umbra can hit hard and knock you down if your agility level is low. Vindasel does not contain many enemies, but there are two traps: a gas emitter and a dropping floor with spikes at the bottom. It is east along the road from the Clavicus Vile's Shrine, and west of Pell's Gate and the Old Bridge. Vindasel is located just southwest of the Imperial City. If you just fast-travel to the Waterfront District, you can swim across the river. When the icon with the cross and dots around it shows up on the minimap, just follow it there.
There are several Welkynd Stones on high pillars both alongside walls and in the middle of rooms. Either jump and reach for them (easiest with a high acrobatics level) or shoot arrows to get them down. You may also use Flare, but it may be difficult to see where the stone went after hitting it. In order for Flare to knock the stone loose, it must be a direct hit (your crosshair must be directly on the stone--not next to it or near it, as Flare's "splash" won't knock it down).
The name Vindasel may be a reference to the actor Vin Diesel, who lived in Bethesda, Maryland, where Bethesda Softworks is based.