For the past few months, I have been playing a Rogue Assassin. Initially, I wasn't completely convinced, but I was wrong. It's a really fun playstyle, with solid single target damage and the ability to clear quests easily with some tricks. It's exceptional defensively, and assassinate is very effective.
Although I am not a big fan of Two-Weapon Fighting and prefer the mobility of other styles, TWF is definitely a valid choice for a rogue. The ability to assassinate two targets at once is a great advantage, and it deals higher damage against single targets. Vs groups, Lotus with stun upgrade and chains is solid.
Regarding the build, perfection doesn't exist, and choices need to be made. I chose to make my assassin Dex-based since I wanted to play the Assassin to assassinate
Here’s a screenshot of my stats in reaper (self buffed).
Shadar-kai Rogue 20, SD (39) GmoF (23) LD (11)
An Assassin build has comparatively lower melee power when compared to other top-tier DPS builds (even with Prowess swap on, I only reach 426). However, this is offset by sneak dice.
Other relevant numbers:
Enhancements
The entire Assassin tree is really strong, but you can't have it all unless you use KtA as a trance or give up 20% movement speed. However, I wanted both assassinate and an area stun that could work even in the toughest content, and Dex wins vs Int.
Assassin's T5 is fabulous, and it's a T5 where you would want to take everything.
Measure the Foe is essential (30 mp, 10 dodge, 5 assassinate DC, 5 to hit), also because it's extremely easy to keep up, just click it before it expires. Knife Specialization is 4 critical units, another must-have, as well as Assassinate if you want to be an assassin
The choice for me was between the hp of Light Armor Mastery and Deadly Strike (which is also very strong). I chose hp for an easier life, but both choices are valid. Of course, you can have both if you give up the Dex trance. Basically, the trade-off is higher tactics DC vs more dps (when Deadly strike is applied).
Feats
TWF line, IC, Precision, Spring Attack line, Nimble Fingers, Improved Sneak Attack, OC, PTWF, Doublestrike, Crush Weakness, Harbinger of Chaos, Scion of Ethereal Plane.
Epic Destinies
Unfortunately, there is no ideal solution for destinies. Shadowdancer is great for a stealth-playing assassin, but its T5 is terrible. It's not just bad, it really sucks.
Also, Dire Charge is unplayable, and to get Lotus, you have to spend 23 points in Grandmaster of Flowers.
While playing a stealth assassin, I don't like to give up Shadowdancer's mantle: sneak speed and 75% concealment, very underrated in my opinion. Plus 15% helpless if you invest in T4.
In the end, I tried various setups, including ones without T5 (T4 and Core 4 in SD, GMoF, and Fotw), and the one that satisfies me the most is Shadowdancer T5, Grandmaster of Flowers T4, and Legendary Dreadnought Core 3.
However, it depends on who you play with. You can sacrifice action boosts and some DCs for spot healing, using DC (Smite for healing support and Crusade between fights) or US as third tree.
Gear Setup
Some considerations on gear... First off, 60 MRR... aaaaargh! I know, it's a bit frustrating, but I don't have mythic + reaper 5 melee power on Mind and Matter rings. Sad Panda.
My personal recommendation is always to base your gear choices on what you have available. For this particular gear set, I have melee power boosts on almost everything, which is great for maximizing DPS. The more melee power you have on items, the more DPS you can dish out!
Following is the gear breakdown.
Overall, I find this setup to be excellent as it maximizes all the key elements: Assassinate (+17+8 -Silver Tongue gives 8, not 7 as stated- +3), Stunning (+16 augment +5 Pendant), Deadly (Flameguard Lenses), Doublestrike (which isn't really an issue for a dagger rogue), Seeker (Tinkerer's Gloves), and Improved Deception, among others.
Although it's missing Quality Deception... you can't have everything.
The Raptor Teeth Necklace is fantastic and can be used as a swap.
For augment sets, I've opted for Cunning Impact (Dex) and Tough Shields (PRR), but MRR Cap is also a viable option that trades 30 PRR for 30 MRR. I usually try to survive with 60 MRR and use Crystalline Gauntlets as my first absorption swap, followed by trinkets.
As for weapons, I prefer to use Dino Daggers. Goldcursed is nice, but with a good Lotus, its use is really niche, in my opinion.
For filigrees, I use Sucker Punch/One Against Many x2, Treachery/Twilight's Cloak x2, Dreadbringer Melee Power x2, Dreadbringer Tactics x2, Dreadbringer Dex, Treachery Melee Power, Bravery Throughout/Shattered, Shattered Melee Power, Shattered Armor Piercing, and Shattered Attack and Damage as my base set.
Additionally, there are the sentient swaps, including Prowess (for action boosts) and Healing Amp (with LGS offhand), as well as the full Dex swap for trance, though I tend not to use it as I find it tedious when there's lag and I also have to scroll Tenser.
Pros
- Assassinate. It's very enjoyable, with the potential to target two enemies per attempt. Based on my experience, 121 DC is enough.
- Lotus. It's my favorite epic strike for TWF. It's fast and effective for both damage and crowd control, with no chance of failing the DC even in tough content. The highest DC I achieved is 147 with boosts and Ancient Tactics on... overkill. High 130s is more than enough.
- Chains. TWF chains aren't as good as SWF chains, but they deal satisfactory damage against helpless mobs with a 53% damage bonus.
- Damage avoidance. Impressive 75% concealment, along with 41% Dodge + Improved Meld and Improved Uncanny. Sky-high Reflexes as well. Unfortunately, Jaunt and From the Shadows share the same cooldown, I have Spring Attack too.
- Sneaky. Fast stealth movement speed, very high Hide and Move Silently, Hide in Plain Sight, and up to 47d6 Sneak Attack damage.
- Some unique debuffs. Bring Darkness and Shadow Loss are great if other party members don't provide them.
- Traps. Obviously.
- Balanced. Even though it's not optimized for raiding (it can be adjusted to take T5 LD, etc.) and not the best build for blasting through quests, I think this assassin build can perform well in any situation, with good single target DPS, and Lotus and chains for groups.
Cons
- Tenser. You always have to scroll Tenser. The duration of 1:06 is quite annoying.
- Healing and self-healing. If you want to heal others or yourself, it's better to sacrifice LD and take DC/US as the third tree. It depends on who you play with...
- TWF. Pros (single target DPS while standing still), but also cons. The mobility of other styles is often missed, even though Lotus and Chains are helpful.
- Capped MRR. If you don't pay attention, you can get obliterated in some situations. But you can live with it. If you don't like 60 at all, you can easily reach 90 by changing augment sets or gear.
- Epic Moment. There's no need to beat around the bush, Shadow Mastery as an epic moment is pretty terrible, especially when compared to Action Hero.
- No rages. Playing a melee without investing in FotW, without Blood Feast, Unquenchable Rage, etc., is still bewildering to me
Feel free to ask for more information.
Much to be learned there, I feel! 
