Twin Fang Strike mechanics?

I'm currently leveling on Terror as a TFS sin and have a few questions on the build mechanics.

Is it a Spell or Attack? Does it get bonus from +Spell %? + Poison/Fire skill %? Bonus from EFSD?

Way of the Spider/Phoenix increase TFS damage?

Shadow Refuge gives x% Phys Damage to Attack; when using TFS, does the x% Phys Damage apply to the attack?

TFS Poison explosion does X-Y psn dmg over 1 second. How does Poison Skill Duration work with this? If I have 100% Psn Skill Duration, does it change to X-Y over 2 seconds?

How does Shadow Dancer passive interact with TFS? Does TFS benefit from Critical Strike? Does claw physical damage increase in anyway benefit TFS?

I haven't put any points into energy this build and I also got Teifling form not knowing it put defense to 0...so I'm probably respeccing soon since I'm dying a lot. Just wanted to fully understand before I do.

Any other advise or suggestions on the build is welcome =D

For reference:

Here is my current Skill Tree: https://skillplan.median-xl.com/planner?id=02guT9kI

Currently equipped items:

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It's a spell triggered from a melee attack. You don't need to hit to proc the spell, but you do need to hit for life/mana on attack to work.

TFS spell damage is boosted by +fire/poison/elemental damage and energy & energy factor. Even if you don't invest into energy factor you still want ~500 energy for double damage. Keep in mind energy factor is not additive with spell damage, it's a total damage multiplier.

Way of the spider/phoenix do increase damage via extra spell damage but you're always better off dumping points into blink/cloak/icon whatever.

Shadow refuge physical damage bonus applies only to the direct melee hit, not the spell. Twin Fang gives +poison spell damage to shadow refuge, so you want to maintain shadow refuge at all times.

Shadow Dancer only boosts the direct physical melee hit. Claw damage is irrelevant for TFS spell damage.

Poison duration increases poison duration without changing dps.

Until you can achieve a decent block, you want use a honorific claw in your off-hand with LOSIM/MOSIM/LAEK/FCR etc... shield itemization for sin is still horrible.

Thanks for the info, really helpful. Will need to level a bit now to dump points in energy since its all in str/dex unfortunately. Shadow Dancer doesn't seem worth it to me if it is irrelevant to TFS. I have no clue why I left Blink as a one-pointer...so much better with longer stun/shorter CD!