[HC, 1.2] CoC/Block Spectral Throw with Vagan's dagger - great DPS and survivability

Typical lvl 75 map Plateau run in HC, 2:30
Random volatile mob encounter
Random elereflect + CB encounter in monster damage map
more to come later

Skill tree for 1.2.x, lvl 92 character
Possible skill tree for 1.3. No block nodes at all, hoping for Rumi's Concoction to remain not nerfed

People often underestimate the importance of high accuracy for a crit build. Accuracy gets checked twice if you roll a crit, if any of those checks fail, you won't get a crit. Source.

But what does it mean in practice? Let's take a look at a quick example, comparing two daggers.
1. We have typical 85% chance to hit, 80% crit, level 20 CoC gem. Our chance to proc a spell is calculated like this: chance to hit^2 * crit chance * CoC chance. So: 0.85*0.85*0.8*0.68=0.39304. Our real chance to cast a spell on the attack is 39.304%.
2. We have Vagan's dagger - 100% chance to hit. Our real chance to cast a spell: 1*1*0.8*0.68=0.544. 54.4% chance.
This means with Vagan's dagger we would have 38.4% MORE procs overall, our effectiveness is that much higher.

More info on topic can be found here.

DPS is quite insane. The more monsters you hit, the higher amount of procs you get. This means both great aoe dps/clear speed and ridiculous single target damage, if this single target is surrounded by at least a couple of mobs. Map bosses get destroyed in a second or two usually. Autocursing with two curses using Herald of Thunder means we: 1) don't spend time to curse, improving our clear speed by a lot, 2) don't use PCoC in the main link because there is no need - we can reliably sustain power charges with Assassin's Mark. Check the video above to get the idea of dps.

Despite crazy damage output, this build has enough tankiness to survive the most difficult encounters. We are using Lightning Coil, Rathpith, Stone of Lazhwar, Rainbowstride and Berek's Grip, all mostly defensive uniques. Our HP pool is pretty high too, 5.3k hp/es total with 187% hp on the tree. The build was meant to be HC viable from the beginning and it definitely is.

Elemental reflect does absolutely nothing, because we have 75% spell block and life leech against frozen enemies on Berek's Grip, and we freeze most monsters. We're also using 2 Surgeon's flasks (Topaz and Ruby) and Atziri's Promise flask for additional reflect mitigation. Don't forget about Vaal Pact, we have it too. See "videos" section for elereflect encounter.

That doesn't mean we're totally unkillable though. Be very aware of:
• invulnerable totems or monsters with this aura, it might cause a huge lag if you have an old pc (like I do), and if there's elemental reflect nearby, you might not be able to use your flasks properly
• one-shot mechanics like slam of Palace Dominus or Maze Vaal
• Corrupting Blood and Volatile monsters. you might not react fast enough and CB will kill you with insta-20 stacks, having Staunching flask is mandatory. Volatile with extra damage mods on hard-hitting mobs is pretty scary too. see the examples how to handle those in "videos" section
• really hard-hitting mobs like big skeletons or giant vaal fallens. if your immortall call isn't up for some reason and you're unlucky with blocks, you might get killed in several swings.
• double reflect isn't pretty. i would not recommend doing elereflect maps in general, but if you do so, be really cautious with monster auras.

Main skill - GGGBBB
Spectral Throw + Greater Multiple Projectiles + Cast on Critical Strike + Ice Spear (4L)
+ Arc (5L)
+ Fireball (6L)
Why these skills? Will tell in the other part of the guide.

Auras/buffs - RBBG
Reduced Mana + Purity of Elements + Tempest Shield + Herald of Ice

Autocursing setup with 2 curses - BBBB
Herald of Thunder + Curse on Hit + Elemental Weakness + Assassin's Mark

Movement skill - GGB
Whirling Blades + Faster Attacks + Power Charge on Critical

CWDT setup - RRRR
Cast when Damage Taken + Enduring Cry + Immortal Call + Increased duration

And then we have one 3-link for using anything we want. My gear has a bit different links (2 CWDT setups), because I had to work with colors I got and because it's really hard to get RRRR colors for Maligaro's Virtuosity.
Optional skills
movement: Lightning Warp (+ Faster Casting/Reduced Duration)
utility: Vaal Haste/Grace/Discipline (+ Increased duration)
whatever else, you can use Portal + Faster Casting + lvl 4 Enhance if you want

• Normal - help Oak and get hp
• Cruel - skillpoint
• Merciless - help Alira and get power charge

You can level whatever way you want, but I personally recommend choosing some elemental spell, Flameblast / Arc / Lightning Tendrils are all good.
Don't try to use CoC setup before lvl 60-70 or if you don't have Vagan's dagger.
Priorities on the tree: hp%, generic crit nodes, block nodes, +30 stat nodes if you lack them. The order of taking nodes depends on two things: what type of league you're playing (HC or SC, you can get more damage earlier in SC) and how comfortable you are with your current hp pool (heavily depends on available leveling gear). Priorities on gear: hp and resists.
(HC note: if you're using flameblast, start being careful around a1 cruel, when elemental reflect first appears. You also want to be fire res capped and/or carry a ruby flask.)

This is how your tree might look like:
Before respec for CoC setup (lvl 67)
After respec (lvl 67)
Lvl 80
Lvl 92 (my current tree)

Disclaimer: this section as some others is WIP, so it might not have every bit of info you might want. Ask questions in the thread.

Gems and quality gems
This build doesn't have any mandatory quality gems, like Incinerate or Freezing Pulse builds. However, GMP might cost you a couple of chaos, CWDT is a quest reward though. Other gems aren't worth mentioning and could be bought for alch each, if you are not getting them as rewards. With that said, it's pretty important to level your main gems on swap, talking about Arc, Ice Spear, Fireball and Cast on Crit here.

Weapon
You are obviously going to need a Vagan's dagger. There are no alternatives. You might get lucky and buy a blue/rare base for real cheap, but multicraft Elreon mod costs 2 exalted as of now, so ready to be used dagger can't cost less than that. Poignard and royal skean are best bases, but that doesn't mean you can't use other ones.

Shield
Rathpith Globe is pretty much mandatory if you want to be safe against reflect. Price varies in different leagues, use poe.trade for reference. Should be about 15 chaos-2 ex range.

Chest
This is where we can save a lot of currency. Two words, Tabula Rasa. Links are extremely important for this build and getting Tabula Rasa is a wonderful choice, especially if you're playing SC leagues. To be honest, I would prefer Tabula over a 5L chest with hp even in HC. So, ranking order: 6L coil > 5L coil > Tabula Rasa > random 5L chest (preferably evasion/ES or eva base for colors). Tabula should be 1 ex or less, random 5L chest - a couple of chaos. Don't go lower than 5L.

Helm
Rat's Nest can be bought for cheap (5-10 chaos if you're lucky), but it's not necessary, you can just use generic life+res helm for 2 chaos, especially if you're playing HC.

Gloves
Maligaro's Virtuosity is pretty much BiS here, but generally you make a choice between them or Rat's Nest. I believe Rat's Nest to be better than Maligaro's at lower lvl and cheaper gear setup and vice versa at high level. I also think Maligaro's cost a bit more than Rat's Nest, so I would just use rare gloves with hp+res here, attack speed is good too if you can get it. 2-5 chaos.

Boots
Rainbowstride allows you to cap your spell block. You can buy some typical hp+movement speed(+res) boots, knowing that you wouldn't be spell block capped (but pretty close to it). You would probably want pure ES base for easy BBBB which you need for single/double autocurse. 2-5 chaos.

Amulet
Stone of Lazhwar. It's mandatory just like Rathpith, but it costs a lot less. Several chaos in SC, a bit more in HC I guess.

Rings
Doedre's Damning is a great choice if you can get it for cheap (it's a lot harder now that they nerfed its drop rate). Otherwise just use a single curse (Assassin's Mark) and rare rings with hp+res, preferably diamond base. 2 chaos each. Hard to tell Doedre's price, standard prices are a bit misleading I think, but in the last HC temp league they were about 15-18 chaos usually.
If you're playing HC and you don't have Berek's Grip yet, you should still be careful with elemental reflect even with 75% spell block. Or just use Life Leech as a 6th link in your Tabula.

Belt
Leather belt with hp+res(+str). 2-5 chaos.

Summary: you can get everything for as low as 3-4 exalted, most likely it's going to cost more, up to 7-10 exalted. Which is still not a lot for a functional crit build.

The build can do most map mods. Half regen and no regen are easy if you have Berek's Grip. Elemental reflect is doable, but too annoying to do. With my current spec I can't really do:
• Elemental weakness maps. I'm not capped there. Can use rare gloves with a lof of resists instead of Maligaros, but I never bother doing so.
• Blood magic maps. I'm not capped there either, because I have to use Purity of Elements.
• -res maps. It's just something almost no one in HC really does. At least consistently. I think it's simply not worth the risk.
Otherwise any reasonable amount of dangerous mods is doable, with current map system it's just not very worth it to do them usually.

I am using legacy Maligaro's Virtuosity and legacy unique Berek's Grip (it can still be dropped with Zana's map mod though). Do I really need them? The answer is no. You can still use usual Maligaro's or defensive rare gloves if you want, it's not that big of a deal. No Berek's means you have to be more aware of reflect packs - 75% spell block doesn't mean full reflect immunity, you are still killable with bad enough circumstances OR you can just use Life Leech in your link (really want 6L for that though), if you're playing HC. This is THE safest (and most boring) choice you could possibly do.

Real huge thanks to my friends horse_with_3_mountains, zamppe and hrodgar09 for making this character and guide possible.
Also to /u/andimjustsittinghere at reddit for assuring me i'm going in the right direction.