Threat issue - Assassin

normally i ask my group to give my a few secs before they start attacking, also there is a damage part to threat generation . your DPS could steal some threat from you since DPS shadows have way more burst damage then tanks. but using slow time and force pull should get them off quickly

Here's what could help:
1) Assuming the spec your talking about is NOT putting talent points into shrouds of darkness (1% endurance per point) but rather 2/2 in thrashing blades and 1/3 in electric execution should help boost threat. note that 3% endurance end game, assuming you get the average health pool of 20k really 3% of 20k is only 600 hp. Not worth it
2) Shock isn't scaling properly with the assasin's training buff you get from being an assasin.  So really don't use shock unless you proc the crit buff to make it worth it.  Only use shock w/o crit buff on the regular mobs to stun them (for dmg avoidance)
3) DON'T BE AFRAID TO PUT GUARD ON DPS!!!! Realy when you think about it, they're the once that benefit from it.  You get the threat, they get dmg reduction.  Of course that doesn't mean put it on dps all the time but depending on your tanking strategy  you should keep guard in plain sight and ready to switch it whenever. (I am a proactive guard  buffer. I will switch it during pulls just to help out heals.)
There are ways around it, just gotta think outside the box! Hope this helps!

Sup guys, new here.   31/0/10  Assassin.      
 
Even doing my job perfectly at level 50, when DPS are doing their job perfectly, they can generate a ton of threat.  Due to the abscence of threat meters and other tools to track output, our DPS work their threat mitigation abilities into their habitual rotations.  I too once played a well known Blizzard game, and was a stealthy melee dps... I learned that my threat mitigation skill, though consuming a GCD, allowed me to live to DPS till the end of the fight.
Bottom line, if you are truly doing all you can with your abilitied, and minding what Darthaggro said above, tell your DPS to cool it.
My biggest problem is pugs rushin in to pull before I do.  Enjoyed seeing a pug assassin DPS get one shot by the first boss in HM BT earlier today.  I didn't even have an ability off, was just running in to pull when he sped out in front of me.   I lol'd and stood on his corpse until the boss died.

Ok I am my guilds Main Tank in many ways as an assasin tank (Hurray assasin tanks) and i never have aggro problems. In fact on the harder fights like Colonel Vorgath in EC im preferred because of my aggro keeping abilities. So heres what i do. Take discharge out of your rotation, it doesnt do enough damage to generate a proper amount of threat. I only use it on AOE pulls.
I run in and use shock right off the bat, then wither. This generates 2 stacks of the buff you need to self heal, then thrash until you proc your shock crit buff. Throw shock out there. Now you have generated 3 stacks and you should cast force lightning. when specced properly i hit on average 2k crits from force lightning. 1.5k-2k which is approx 3k aggro every tick! Use wither on cooldown and keep your dark charge up. Keep this rotation going and youll be fine. Dont use lacerate or discharge. And throw in saber strike when your low on force. Also if you need a boost in threat, when your about to stack your 3 stacked force lighting (force lightning should never be used unless you have built up 3 stacks) pop recklessness and a relic. Youll crit per tick at almost 3k. And dont be afraid to throw out your AOE taunt on single mobs, it functions the same way and itl keep your aggro up!
I always shield dps on boss fights. As healing generates 50% less threat than damage you shouldnt have to worry about a healer pulling. Pick your strongest dps (typically a mara or a sniper) but make sure they stand close enough to you to recieve the shield. Switch to shielding healers during trash pulls.
Give this a shot and let me know how it works out for you! If you have any other questions let me know.
 
Assasin tanks ftw!