Soul Siphon fires when you kill something. On Faded Ascendant Relic it reads: Kills charge your Ultimate by 2 points.
Word for word it is Kills charge your Ultimate by {value} points, and the number in it grows with the gear: 6 to start, 0.4 more for every item level.
8 bases carry it, and 7 Uniques are built on those: A Patch of Night, The Overlord's Retinue, Morningshroud, The Sky's Own Star, The Glassblower's Breath, Thunderroc's Halo and The Noonday Crown.
Awakening can reach it too — 24 spirit paths pick it up at Spirit Level 3: Hungerbound, Nullbolt, Hollowcall, Eclipseedge, Dusktyrant and Nullfang and 18 more.
Gear that carries it
8 bases
| Base | Slot | Value | iLvl | Found in | |
|---|---|---|---|---|---|
 | Faded Ascendant Relic | Relic | 2 | 1 | DEMON |
 | Dim Ascendant Relic | Relic | 2 | 15 | DEMON |
 | Ascendant Relic | Relic | 2 | 30 | DEMON |
 | Kindled Ascendant Relic | Relic | 2 | 45 | DEMON |
 | Radiant Ascendant Relic | Relic | 2 | 60 | DEMON |
 | Eternal Ascendant Relic | Relic | 2 | 80 | DEMON |
 | Eternal Voidhalo Relic | Relic | 3 | 80 | NIGHTMARE |
 | Eternal Godspark Relic | Relic | 3 | 80 | MORTAL |
Uniques
7 Uniques
A Patch of Night
Starcaller Relic · drops anywhere
The Overlord's Retinue
Kindled Host Relic · drops anywhere
Morningshroud
Eternal Ascendant Relic · drops anywhere
The Sky's Own Star
Eternal Starcaller Relic · drops anywhere
The Glassblower's Breath
Eternal Weepingglass Relic · Cinder Wyrm
Thunderroc's Halo
Eternal Voidhalo Relic · Thunderroc
The Noonday Crown
Eternal Godspark Relic · The Dawn Matron
Spirit paths that reach it
24 paths · value at each Spirit Level
| Path | Family | Affinity | L3 | L4 | L5 |
|---|---|---|---|---|---|
| Hungerbound | Relic | Void | 35 | 55 | 90 |
| Nullbolt | Wand | Void | 35 | 55 | 90 |
| Hollowcall | Staff | Void | 35 | 55 | 90 |
| Eclipseedge | Sword | Void | 35 | 55 | 90 |
| Dusktyrant | Greatsword | Void | 35 | 55 | 90 |
| Nullfang | Dagger | Void | 35 | 55 | 90 |
| Hollowlitany | Tome | Void | 35 | 55 | 90 |
| Netherkiss | Dirk | Void | 35 | 55 | 90 |
| Umbragaze | Helm | Void | 35 | 55 | 90 |
| Flareband | Ring | Fire | 35 | 55 | 90 |
| Iceband | Ring | Frost | 35 | 55 | 90 |
| Sparkband | Ring | Storm | 35 | 55 | 90 |
| Nethersigil | Ring | Void | 35 | 55 | 90 |
| Goldband | Ring | Holy | 35 | 55 | 90 |
| Redband | Ring | Blood | 35 | 55 | 90 |
| Festerband | Ring | Poison | 35 | 55 | 90 |
| Obsidianband | Ring | Stone | 35 | 55 | 90 |
| Emberidol | Relic | Fire | 35 | 55 | 90 |
| Rimeidol | Relic | Frost | 35 | 55 | 90 |
| Tempestidol | Relic | Storm | 35 | 55 | 90 |
| Dawnidol | Relic | Holy | 35 | 55 | 90 |
| Crimsonidol | Relic | Blood | 35 | 55 | 90 |
| Venomidol | Relic | Poison | 35 | 55 | 90 |
| Graniteidol | Relic | Stone | 35 | 55 | 90 |