Yeah, Soul Mastery is a pretty nice pool.
Gloom is a very good ranged single target attack, fairly quick cast time with a bit heavier damage than most due to it being a DoT, but thankfully the DoT doesn't take long to complete.
Soul Tentacles is decent AoE damage albeit with a lengthier DoT, in addition to the obvious advantage of mass immobilization (with a reasonable cast time).
Dark Obliteration is also a solid AoE - nothing spectacular, but it pulls its weight. The -tohit may help a bit if you're not softcapped, or in cases of enemies with +tohit or -def.
I've not used the patron version of Darkest Night, but it's a great power in the Dark Miasma set. I imagine even with the significantly lower debuff values that it's still pretty useful here too - even if you don't need the -tohit, the -dam will take the edge off any attacks that get through, and it's a good tool for corner pulling entire spawn(s).
I've no experience with the Widow pet, so can't really comment there, but I've not heard of it being significantly better or worse than any of the other patron pets.
Leviathan is the popular choice due to the synergy between Venom Grenade and Bile Spray (the late addition of more -res in Arctic Breath didn't hurt its case any either), but that aside Soul would make for a solid choice too.
I have no decent experience with the villain patrons, but knowing main point of the powers I can try to explain something important! 
Soul Mastery looks pretty mean for a Crab. Soul Tentacles (AoE Immob), Dark Obliteration (Targeted AoE), and Darkest Night (toggle -Dam -ToHit), mixed with your Crab powers like Venom and Frag Grenade, and Suppression, you could easily be a AoE beast. Omega Maneuver (some take it, some dont, I plan on it.) could provide even more AoE Death, but that's your decision to take it or not.
All the other Patron sets are the same power template, except Soul is the only one with a toggle. Mace Mastery looks appealing to me, with the last power getting another Arachnobot that I have perma, in Mids 
My recommended attack chain, with Soul Mastery, would be:
Soul Tentacles -> Darkest Night -> Venom Grenade -> Dark Obliteration* -> Frag Grenade* -> Suppression -> Finish off whatever's left.
* = You can switch the two powers, doesn't really matter.
IMHO, YMMV,
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If you can stand the animations, Leviathan Mastery is actually a good choice for crabs. Arctic breath and bile spray are both large cones and while they have fairly longish animations, arctic breath does a 15% resistance debuff that you can stack on top of venom grenade and bile spray is all toxic damage, which means you get double the debuff from venom grenade. So a combo of venom grenade -> arctic breath -> bile spray will result in bile spray hitting enemies that have been debuffed by 55% to resistance.
If you can't stand the animations (and they do look kinda gross) then Soul is a good alternative for crabs as mentioned by the posters above.
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Howdy folks,
I've gotten a crab to 50 already and went with the much given advice to take leviathan mastery because of the cones. It's enjoyable, and I <3 being an AoE/cone death machine. I'm working on my second soldier now. Conceptually, I've got my eye on soul mastery but I've never actually seen anyone take it. The combination of soul tentacles, dark obliteration (which I've never actually seen animated), and darkest night look like decent powers, but I have no idea if it's a good idea. Any advice?
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