To get us started, here's a scythe-wielding Assassin I've been trying out the last day or so. I've seen a few similar concepts around, but these are the skills which I've found to work fairly well so far.

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1. Radiant Scythe (EotN)
2. Mystic Sweep (N)
3. Eremite's Attack (N)
4. Critical Agility (N) (PvE)
5. Way of the Master (EotN)
6. Critical Eye (F)
7. Aura of Holy Might (F) (PvE)
8. Way of the Assassin (N)
Basically it makes use of the high max damage of a scythe by providing an extremely high critical hit chance. These critical hits provide a high level of energy management (4 energy returned for each crit hit), and maintain Critical Agility. Aura of Holy Might is not essential, but will significantly increase the damage that your scythe attacks are doing. Critical Agility helps make your assassin a little less fragile with an armour bonus, and also helps to bring the attack speed of your scythe closer to that of daggers. You should have pretty powerful DPS even from your normal attacks, against base 60 armour targets I was averaging somewhere around 66 damage per second, that was with a non-customized non-max white scythe though. I would expect that to improve quite a bit with a proper weapon.
The choice of attack skills is somewhat arbitrary, but I'll explain why I like the three I listed. Radiant Scythe can hit harder than almost any other equivalent attack skill, provided your energy isn't too low. Mystic Sweep and Eremite's both have a 3/4 sec casting time, rather than acting as true attack skills. This means you can cut out the cooldown from your previous attack and jump straight into them. By using the three skills in quick succession, they function as a bit of a spike, since you're hitting pretty hard to begin with. Healers shouldn't have too much time to react if you use them right.
Having 5 skills to use just to prepare for battle is fairly taxing on your energy, but it's sustainable since it should be shooting right back up once you start hitting. You can probably cast Way of the Master before going into battle, since its duration is so much longer than the others. The same could be said of Critical Agility, since it will last 18 seconds and you only have to critical hit once to renew it. The only other skill with a casting time is Aura of Holy Might, which you'll want to wait until you're in battle position for so that you hit the enemy with holy damage from it. I'd recommend Zealous insignias though, or you'll probably be running a bit short. As far as the rest of the armour goes, I chose a Critical Strikes headpiece with a minor Crit Strikes rune, a vigor of course and attunement for the other rune slots. Stick with whatever suits you, though. Weapon choice isn't too important either. My favourite would probably be a sundering scythe, either of enchanting or fortitude. 15^50 or 15% while enchanted are both fine choices, you could also use a 'while in a stance' insignia if you make sure to keep Way of the Assassin up. Zealous might work well, but I'm not sure you'd need it. Vampiric might be good if enemies are grouped together tightly, you'd want to carry a secondary weapon for when you're out of battle though.
Depending on what you want to accomplish, you can swap things around a bit. Critical Defenses could work well if you're taking too much physical damage, probably in place of Aura of Holy Might. You could also try adding some conditions into the mix using something like Sharpen Daggers, in which case replacing an attack skill with Malicious Strike would probably be a good choice. Assassin's Remedy might also be useful, in areas with lots of conditions. Experiment a bit to see what you like, there's a certain amount of flexibility in the structure of the build.
As for weaknesses, there's the usual problems with melee builds such as snaring, blind, anti-melee hexes, etc. It doesn't really have any way I can see to end stances, but you could probably play around with finding a way to become unblockable. Enchant removal would strip away its effectiveness a bit, but it'd have to be pretty heavy enchant removal to cause serious harm. The usual, in other words.
Anyway, feel free to post comments or suggestions or whatever. And post some of your favourite builds in other topics, whether they're really effective or unusual or just because you like them.
