Revenant ether is always dropped by revenants in the Revenant Caves. The bracelet of ethereum has a feature to automatically absorb ether from defeated revenants.
Revenant ether is used to charge the bracelet of ethereum, which reduces damage from revenants by 75%, with 1 charge being consumed per attack. It is also used to charge Craw's bow, Thammaron's sceptre and Viggora's chainmace, costing 1 charge per attack (whether it hits or misses).
Revenant ether is always dropped on death in the Wilderness, even when it is stored as charges within the weapons or bracelet of ethereum.
Disassembling various uncharged items will give the player ether:
||Bracelet of ethereum (uncharged)
||Craw's bow (u)
||Thammaron's sceptre (u)
||Viggora's chainmace (u)
For an exhaustive list of all known sources for this item, see here