Oblivion:Void Salts

The ingredient Void Salts is primarily collected from dead Storm Atronachs.

Three samples of Void Salts are necessary to activate the Atronach Altar in the Wizard's Tower official plug-in, if you wish to use the altar to summon a permanent Storm Atronach companion.  (Fire Salts or Frost Salts will instead provide a Flame Atronach or Frost Atronach, respectively).

Ingredients
5 guaranteed samples can be found in the following locations (magenta crosses on map):
1 sample is in The Dividing Line in Leyawiin
1 sample is in the Bravil Wizard's Grotto 
1 sample is in the Bruma Mages Guild Living Quarters
1 sample is in Sinderion's Cellar in the West Weald Inn, Skingrad
1 sample is in the Crypt of The Night Mother in Bravil
It can also be found randomly in the inventories of alchemy vendors (starting at level 20) and in some Conjurer and Oblivion loot chests (starting at level 17).

Alternatively, the Vile Lair plugin may be used to obtain Purgeblood Salts for the purposes of Alchemy since both Void Salts and Purgeblood Salts share the same Alchemy Effects. An unlimited supply of Purgeblood Salts may be obtained by exploiting a glitch with the Purgeblood Crystal Formation.

However, the Purgeblood Salts cannot be used in the Wizard's Tower Atronach Summoning Altar to summon an Atronach Familiar despite having the same Alchemy Effects as Void Salts and sharing the same inventory icon as Fire Salts. Also be careful not to mistake the Purgeblood Salts for Fire Salts and sell it off by mistake or drop it accidentally. In which case, installing the Unofficial Oblivion DLC Patches will change the Fire Salts icon used by Purgeblood Salts to a different Purgeblood Salts icon that better matches the item model.

Creatures
Standard Storm Atronachs (not summoned ones) and The Sunken One are the only sources of this ingredient.  Storm Atronachs are commonly encountered in Planes of Oblivion at levels 19 and higher. As with other daedric creatures, in Tamriel the best place to look for Storm Atronachs is in Conjurer Dungeons (circles on map), where they may spawn at levels 19 and higher.

Trying to find creatures with Void Salts before level 19 is challenging.  Only one low-level source exists:
The Sunken One appears in Sandstone Cavern (red circle) at all levels.  It spawns during the quest The Sunken One and will respawn after the quest is completed.
Other sources are primarily restricted to locations specific to the Main Quest.
During the quest The Battle for Castle Kvatch, there are 9 courtyard creatures and 19 town creatures who may be Storm Atronachs starting at level 18.
The caves in Mankar Camoran's Paradise (during the quest Paradise) contain 12 daedra who each have a 38% chance of being a Storm Atronach starting at level 16.
The Eternal Gardens in Paradise contain 8-14 daedra who each have a 25% chance of being a Storm Atronach starting at level 17.
The daedra spawned during the Defense of Bruma quest each have a 15-20% chance of being a Storm Atronach starting at level 16.

Outside of the main quest, the best places to look for Storm Atronachs are:
Fort Teleman (purple circle) is guaranteed to have a single Storm Atronach (the boss-level Daedra) at levels 17 and 18.  3 are likely at levels 19 or higher.
Arkved's Tower, Fathis Aren's Tower, Fort Rayles and Sardavar Leed (blue circles/squares) are each guaranteed to have a single Storm Atronach at levels 19 and 20.  They may also have additional Storm Atronachs at levels 19 or higher.

thumb|left|500 px|Locations of Storm Atronachs and Void Salts