[3.9] Vortex Occultist

Weapon:
Xirgil's Crank (Staff) - Great and cheap staff, which when combined with Rumi's and couple of nodes on the passive tree give us quite a bit of ES sustain as well as damage. Probably my favourite staff for a build like this.
Spine of the First Claimant (Sceptre) - DPS-wise the best option out there. Also fairly cheap. Doesn't give much when it comes to sustain though. So you probably want some cold damage leech on either your rings or from the passive tree.

Shield:
Ahn's Heritage - Great and underappreciated shield. Gives a bunch of +max resistances and a lot of life. Only problem is that you need to figure out how to get strenght to wield it.

Body Armour: I honestly think that a rare body armour with Life and resists is all you need, but unique options I personally like are:

Belly of the Beast - Not really used too much today, but still it gives you pretty much everything you need for your body armour; Life and resists. May be hard to get socket colours though.
Loreweave - We are wasting a bunch of stats on this, but it helps with the +max resistance (Do NOT use this with Ahn's Heritage), life and some crit so that we proc our EE more easily. Easier to get colours on as well.
Shavronne's Wrappings - More expensive usually. Allows to transistion to Lowlife version of this build, which has some more dps. I honestly don't see the need for this when leaguestarting, but hey, the option is there.

Boots:
Sin Trek - A HUGE chunk of ES and Evasion. Not my favourite for hybrid builds, since I like having life on my gear as well, but when it comes to uniques there aren't that many acutally good options.
Steppan Eard - Mostly a QOL pair of boots, not as much ES as Sin Treks, but the fringe benefits can be fairly nice, especially if grouping.
Rainbowstride - Real nice pair of boots for the Xirgil's version. Gives spell block chance, which helps proccing your Xirgil's. As well as some resists. Not that much eHP though.
Shavronne's Pace / Gambit - Very nice and cheap pair of ES boots that last for a long time on you. Especially if you can get your hands on Shavronne's Gambit, which has a VERY nice 1% ES regen. Giving you even more sustain.

Helmet:
I honestly think that Rime Gaze is pretty much mandatory, but:
Devouring Diadem - Is a very nice alternative, Saves you a bunch of points on your passive tree and gives a lot of sustain. These things tend to be very expensive though, since only the mastermind drops them.

Flasks:
Rumi's Concotion - Helps with Xirgil's but not that much with other variants. Is nice to have with Ahn's Heritage.

Rime Gaze - Allows you to clear the game with only a fourlink, freeing your body armour slot (so that you can wear even Kaom's Hart if you want to.)
It also gives insane amounts of Energy Shield, which helps with our Eldritch Battery + MoM setup.

Wise Oak - A HUGE damage boost (and defensive one as well if you somehow manage to triple cap your resistances, but you should just concentrate on having your cold resistance capped higher than the other two.)

Witchfire Brew - Another huge damage boost, but don't slot this in until you get your second curse, this will overwrite your frostbite until then.

Other than that, I don't think there really is any need for other specific uniques and even the Rime Gaze is fairly fringe case as you can very easily just run full rares and magic flasks.

Life -> Resists -> ES -> Cold Damage -> Extra stats you may need (Str / Dex)
Life -> Resists -> ES -> Cooldown Recovery (Belts and such.) -> Extra stats you may need (Str / Dex)

So nothing too special and even the CDR is more to the QOL side of things and totally not necessary at all. You just want your ES to be around ~600-700 and the rest goes to your life and resists and stats.

Vortex is a instant cast, cooldown based aoe spell that causes an initial small hit and then a cold DoT for its duration. We're trying to get everything out of this DoT to pretty much melt the maps.
We're supporting the skill with Cold Snap, which generates us Frenzy Charges, Frost Bomb, which drops enemies cold resistance and orb of storms/ball lightning/storm brand which helps us proc our Elemental Overload.
Two curses, frostbite and despair (from our witchfire brew). Being an occultist causes cursed enemies to pop in satisfying purple clouds and help us clear.
Malevolence helps with our dps as well and is the only aura we run.
Other utility stuff we can implement are a third curse, double golems, constant shock (and chill) via skitterbots, nice movementspeed tech via Phase Rush
Frostbolt is also an option if we don't want to cast Vortex in super melee (You can cast Vortex on frostbolt projectiles and it will follow them, turning the skill into a ranged version), but since this build is so tanky, I see no reason to implement it to the build.

Our defensive layers consist of an OK amount of flat Life (I got to ~4-5k without too much trying) and ES with Eldritch Battery + Mind Over Matter -combo, which gives us an insane amount of mitigation thanks to our ES regen and just being an occultist.
We also don't need a cold immunity flasks, thanks to us being an Occultist.

When clearing your shouldn't need to press anything except Vortex to kill most packs, you can activate Vaal Cold Snap when it is all juiced up to help with this. If you encounter a particularly tough rare monster, drop a frost bomb and that should help
when combined with Vortex.
When bossing, just throw everything at it: Your lightning skill to proc EO, Frost Bomb to lower their cold resist, cold snap AND vaal cold snap to help with dps and then just spam Vortex when you're under them. Keep the flasks up and you should be fine.


[If you want to use a non-5/6 linked or Kaom's Heart as your chest piece, you can VERY easily socket your Vortex on your Rime Gaze and play around with other gems to fit them on other pieces of gear.]

Kill All (or if you feel like you're going to have problems capping your resists, take Alira. Not the most optimal choice, but can help. We will be wasting the crit multi roll in that case.)

Brine King or Lunaris or Solaris
Brine King - To help with stuns, since our ES is protecting our Mana instead of our life, we will lose the native 50% stun resist.
Lunaris - If you plan on just running through the maps very fast.
Solaris - If you plan on bossing.

Minor can be whatever you feel like, I like Ryslatha. Physical damage and bleeds aren't fun.

You can go pretty much to either direction, I like Void Beacon -> Frigid Wake -> Profane Bloom -> Malediction
For more tankiness swap out Malediction with Vile Bastion and path to the +1 curse on the passive tree. You kinda have to give up some damage for this though.

Level with Frost Bomb and Freezing Pulse until you get to Vortex and Cold Snap in act 2.
Keep the Freezing Pulse in your back pocket until Vortex starts to feel good to use or you get your Spine of the First Claimant. The CDR on Vortex level 1 is kinda horrid, so it may require some getting used to.
If you come across any large packs of weak monsters, it may even be better to clear them with frost bomb and cold snap (which you can spam as long as you have Frenzy Charges it generates.)

Make your way to Breath of Rime first and then depending if you're dual wielding or not, grab Dark Arts. Then it is just the matter of cleaning up the left side of the tree, get the Spiritual Aid (which is really good on our build, since it directly scales all of our damage)
It can be dropped if you feel like you're running out of passive points, but still, I quite like it for DoT builds that path around the left side of the tree.
Prioritize life on the tree and you should be golden. You can just run through the main story without too many problems.
Unless you get your hands on Enlighten 3 or 4, you will be needing all the mana reservation I have allocated. Sovereignity and the one point under Chaos Inoculation allows you to run your Blasphemy, Malevolence and Skitterbots / Arctic Armour reserving
almost all of your mana, which is very nice.

Get to the Eldritch Battery when you have around 200-300 ES even if you were to swap your gear around. Start grabbing ES regen nodes and Wicked Ward to counteract the regen speed penalty Rime Gaze gives.

Vortex -> Cold Snap -> Damage Supports -> Curses -> Golems -> Utility Skills and auras

Mapping - If you manage to get a white socket instead of one of the green ones on your chest, feel free to swap between Swift Affliction and Increased Area of Effect.
Otherwise you can swap Controlled Destruction with Increased Area of Effect

Level 74, I ran out of time before league started since I was doing so many tests with other gear and respeccing and such. More gear options and such are coming soon (tm). [Aka when I manage to level my character in league!]

-More detailed gem explanation
-More detailed gear explanation
-Videos of endgame (bosses and red maps, as soon as I get there in league)
-3.9 updated gear in case I figure out some upgrades, though I didn't really see anything on the reveals or patch notes.
-Anything that the comments may notify me of and adapting to suggestions!