Darkness Enthroned, Stun Immunity, and getting to skip Unwavering Stance
TLDR: Wearing Darkness enthroned, put 20% stun avoid + 10% stun avoid implicit jewels in = 52% per jewel. Be stun immune + random jewel stats. Take arcane swiftness/chest craft avoid stun/implicits on non-belt jewels to get 100% avoidance from one jewel and start using the other jewel for best stats or ailment avoidance. Save 5 points from Unwavering stance, making up for whatever stats else a real belt would give. Repeat for ailment avoidance implicits and rolls on abyss jewels. Be ailment immune.
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So I've had a problem for leagues now: I like to play some casters, channel some spells, probably be hybrid or CI or MoM and not really have a ton of hp for stun protection, so I usually 'waste' 5 points past the very nice Glancing Blows/block section just for Unwavering Stance. I really don't want to play a non-stun immune character, especially when I'm doing stuff like channeling Stormbind into a mob's face. I've had a presence of Chayula in SSF before, but sometimes I really want to use Foible/gem neck. Stun sucks, but so does not evading. It takes away Flesh and Stone or any other blind as a defensive option. Random bits of evasion on gear don't work, nor enfeeble's accuracy debuff, nor did pre-nerf Jade Flask of Evasion give a cozy 40% evasion.
I found this completely by accident when I wore darkness enthroned and some jewels as the only belt I could equip on a new character at lvl 1 and realized the abyss jewel avoidance rolls went up to 20% (since 3.14 I guess?), and any harvest jewel implicit is multiplied by the +75% effect from the belt. The implicit rolls are pretty common in harvest and not that hard to hit a couple random avoid stun/specific ailment rolls on abyss jewels, regular jewels, and clusters. Tujen sells tons of 83+ abyss jewels that can have the best stat rolls on them, so I buy all his Hypnotic Eyes to reroll since spell damage is fine too.
Lets check the top end with best case jewels, say 40ES, +10 resists, 20 stun avoid, 25-34 cold damage with a shield, 10% stun implicit. Two of those with the +75% is 140ES, 35 All resist, 104% stun avoid, 103 flat spell damage. If that resist is Int, slapping that into my PoB is 100 more ES and 7% more damage than a max roll max catalyst bated breath. If its a triple spell damage jewel I show +20% damage from my belt slot. So I'm going to have far worse jewels than that, but the 5 skill points saved are more than going to make up for that. The bar is pretty low for jewel quality to be worth it with 5 extra skill points, mine just have one or two other relevant stats at middling tier and it's still better than some nuts belt Rog gave me.
This can be extended to ailment avoidance too. Since the rolls only do one ailment, I had to pick and choose and supplement, but I found a pretty low opportunity cost setup so far with my SSF gear:
Chest craft stun/ailment avoidance 20%, arcane swiftness for 20% Anoint elemental focus (or crystal skin but I don't have the oil) to 50% ailment/40% stun. 30% stun jewel in belt gets me to 92%, small stun roll on an abyss on my tree with good stats finishes that. Using a poor 12% shock avoid roll with no implicit in belt as my second because it otherwise has nice rolls, landed 10% shock avoid on Energy from within, and Insulated on a small lightning cluster to be shock immune. I happen to be trying Tasalio's sign as part of a push to enable trinity with a pure lightning spell (hey pob think's its more damage, its more damage, trinity is real good) and that has 50% chill avoid. Crafted freeze immune on boots. Those will probably be a Dream Fragments someday instead. Ignoring Ignite, using Mephod pantheon for some less duration, and an ignite removal flask.
There's a lot of ways to hodgepodge together 100% ailment avoidance but jewel implicits and being able to get huge chunks of a specific one on belt really helps. This will probably become more relevant next patch if they keep hammering at the full immunities and moving towards individual immunities. I could see like a cold occultist use this for ignite/shock and stun avoidance to swap Vile Bastion for Bloom/Malediction, a swap commonly done via Presence of Chayula.
This is pretty flexible, very SSF viable, and I'm sure a trade person could make some nuts belts...if headhunter didn't exist that is.
edit: You don't technically need the belt, it could be a Stygian + 3 tree jewels, but that +75% stats for two jewels turns out to be a lot of stat, probably worth those tree jewel slots being something else.