Since the recent update completely changed how Assassin's work in AoC, I will post some new Assassin builds designed for different reasons.
Raiding/PvE Build: This is the way I have Callesta feated as of right now. The damage done overall is tried and true high dps. The idea here is high volumes of sustained dps which is great for top damage in long fights such as raid bosses or Khitai hardmode boss fights. I make most use of the Lotus tree here to secure the miasma DoT effect and the AoE fear, as well as kidney shots. I ignore most of the corruption tree outside of buffing my critical damage; and spec well into the general tree for good anti-hate abilities that will prolong your dps output without killing you in raids. Facestab is the trick here, relying on it for your bigger criticals and highest burst damages while using your slow death strike+lotus coated dart in between grim corruption+face stab for the most damage.
NEW Raiding build: This build ignores both of the soul fragment combos (Death whirlwind and grave's blossom) in favore of both miasma abilities and extra armor/spell penetration effects; it also stacks up offhand and crit rating/damage buffs, and reaches for some more anti-hate abilities. This is a great build to use during lengthy raid fights and while it lacks the AoE fear of the questing/raid build listed above, it still provides the serious CC and damage firepower required for living in open world questing in some of the harder zones of khitai.
CC Oriented PvP build: This build makes use of the Corruption tree's ability to buff your melee attacks, debuff your enemy's defenses and do high short bursts of damage which makes them ideal for fighting PvP, while also going out of it's way to grab the best crowd control (CC) abilities a sin can have in their arsenal to ensure pvp victory. High burst damage ensures an early advantage on your foe, letting you take advantage of the natural assassin abilities that do quality damage over the remainder of the fight. The charging ability, and natural physical defense debuff associated with their attacks makes them a very fierce and competitive fighter in pvp overall, but does not mean they aren't viable in pve instances/raids and solo questing. Rely on Avatar of death to break CC's and to ensure you survive overwhelming PvE pulls; and land grim corruption+soulstrike for your essence detonation effect while also combining slow death strike with face stab for a near instant pvp kill on cloth armor classes; rely on slow death strike+soulstrike miasma for a swift and prompt stun on classes that get the jump on you or won't go down as easily as a clothie.
Hybrid Raiding/PvP build: This build utilizes the best benefits from both corruption and lotus trees, ensuring good damage done during raids, plus the debuff and burst damage properties of corruption. It also makes use of several CC abilities, but ignores some viable components for anti-hate. This hybrid model assassin is capable of powerful damage in raid or pvp, and will be a true combatant no matter the situation. The lack of anti-hates may hurt in some situations, as will the lack of general feats for armor or protection; but the idea here is not to prevent death, but to bring it fast and furious upon NPC and player alike! With both Death Whirlwind and Grave's blossom in your arsenal, you'll shred enemies by doubling them up with a full force Death whirlwind, then a Lotus Extract to bring up your soul fragment count, only to land Grave's Blossom right after. And doubling up your miasma abilities with Grim Corruption+Soul strike and Slow death strike+Lotus dart will ensure you can top any raid parse when combined with the aforementioned Death whirlwind and grave Blossom attacks. And in between don't forget to throw in some face stab combos when other miasmas are on cooldown; with both buffs feated, it will mean nearly instant death on any NPC or clothie class in PvP if used after a grim corruption or slow death strike combo.
The defensive offense build(great for new 80's in khitai!): This build is especially useful for undergeared new level 80 assassins who plan to roam in Khitai. The emphasis here is on surviving a bit better than the usual assassin, while still kicking some butt. Speccing the general tree for a mix of both armor and protection helps but feel free to move the 2 points in the protection feat over to the armor feat. Obtaining both cat's paw and dull pain will bring your assassin to a point where they will nearly take zero damage from any attacks for a few seconds, more than long enough to CC the NPC attacks and land some of your heavy blows which come in the form of both miasma abilities (Lotus dart's DoT with Slow death strike and Soulstrike's essence detonation with grim corruption) as well as Death whirlwind.
*These builds may be followed, but it's always better to use them to get ideas and alter them for your own style of play*