Foreword: This is an update for FG of the classic build back in the day posted by a Chinese player (I think): https://www.youtube.com/watch?v=kZoGNKZZFJo
Sadly when Ashes of Malmouth came out this build was no longer decent as there were not conversion options for the caster/melee hybrid.
Thankfully since then, and with FG, there are several conversion options, good RR, movement runes and items which can support this playstyle again, and it is better than before 
The Build:
(potato computer really doesn’t do this spectacle justice…) SR 65: https://www.youtube.com/watch?v=xkQ7VqzskFE&feature=youtu.be
HC players would be wise to take aether clusters and/or consumables for SR 65. SR 50 is much safer.
Grimtools: https://www.grimtools.com/calc/4ZDEvq1N
My gear in the video: Conversion gear in the video was 25 on head (average), 35 on body (high), and 40 fire /55 vit on belt (below average). High rolls are not required.
Item notes: MI Patch 1.1.7.0 will be a great help for farming a good Bloodsworn Vestments (Stun res affix is ideal). Craft the head and boots at Steelcap for stun+freeze res. I aimed for 2.7k DA and dumped then stats into Spirit.
I hope you kept all the Voidblades that dropped 
Bloodsworn Vestments can be farmed in Darkvale Village and/or Blood grove. They only drop from Doomsayers or Cultist hero. Highest drop rate, of 50%, is from Zaria (Darkvale Village Boss) and Bolvar (blood Grove). Fortunately there are quite a few Monster totem spawns if you want to farm those at the same time.
Non-MI alternative for the shoulder can be Voidsoul Mantle.
Relic completion should be Destruction or Second Rite.
Energy: Is not a real issue thanks to significant energy leech from Arcane Spark, Viper and Revenant. Make sure to melee enough for it.
Gameplay: Movement rune in, hold left click (Fire Strike), Sigil, Flashbang, Hellfire mine, Solael Flame, Doom Boltx2, Movement rune, Sigil etc.
Hellfire mine is very important, if surrounded you may need to plant another in a different position, make use of the 6 maximum you can have out.
Take note of the small AoE size of Entropic Presence item skill. The damage reduction is needed so try not to take large damage at range (e.g Aleks Meteor).
Levelling guide:
This build can be levelled (with some extra Doom Bolt proc) using certain gear. Use Bladesworn Talisman for Dual-Wield trait, use x2 Bloodsworn Sceptre (buy from Benevald in Blood Grove with of void affix, refresh vendor by examining 2 vendor in homestead then rift back). Rhowari Cord has doom Bolt proc on it, Gloves have the “of dark Intent” affix which also have doom bolt proc on. Depraved rings have a lower level version if you kept/have them.
Other useful MI for levelling: Bloodsworn Vestments, Rylok Mark or Lagoth’Ak Voidbinding.
Quite a few useful Faction gears (not exhaustive list): Solael void head, Malmouth void or tainted Pauldron, Solael void vestment.
Making good use of all the above can provide significant damage for main campaign farming.
Use caster gear until lvl 25.
LvL 3: Occultist use bloody pox.
LvL12: Spec out of Bloody Pox. 1 in Sigil and all the rest into Destruction.
LvL 14: Maxed Destruction.
LvL 16: 1 into Doom Bolt.
LvL 23: Start maxing Possession.
LvL 25: Maxed Possession. One point into Solael Witchfire. Craft Bladesworn Talisman and dual wield Bloodsworn Sceptre asap.
LvL 26: Unlock Demolitionist and put one point into Fire Strike.
LvL 30: One point into Explosive Strike. Take Vindictive Flame and Flame Touched too.
LvL 35: Symbol of Solael and Arcane Spark are now components for damage and energy restoration
LvL 40: Six points into Blast Shield.
LvL 41: Sigil of Consumption at lvl 4.
LvL 48: One in Thermite Mine and proceed to max Hellfire mine.
Begin investing in whatever you like from here on out. Sigil will give more life leech, Doom Bolt for more burst damage, second rite more +% damage, Solael Witchfire for attack speed etc. Flashbang fumble shouldn’t be needed until late game (post 80 or so). Once you unlock Hungering void, investing in Blood of Dreeg and/or Vindictive Flame helps reduce the health cost.
Devotion progression: This setup is pretty straightforward. Veterans might want movement speed first. Newer players flame torrent.
After Flame Torrent:
- Hawk.
- Ghoul or Jackal.
- Eldritch Fire.
- Ghoul or Jackal.
- Viper.
- Revenant.
- Sailor’s Guide, Eel, Hound, Solaen Watcher.
- I recommend damage passive stats of Dying God first then Aeon Hourglass and final point into Hungering Void.