This is a bit of a two parter, as the Assassin suffers from poor power selection, ridiculous fragility and a terrible striking mechanic. The power selection issue I'm addressing beyond this revised profile.
What I've done so far; let me know what you think:
Hit Points & Surges:
Hit Points At 1st Level: 12 + Constitution Score
Hit Points Per Level Gained: 5
Healing Surges per Day: 7 + Constitution Modifier
Why? Increased to standard striker levels to bring the Assassin's staying power up to par with other strikers. Surge count increased to Executioner levels.
Assassin's Shroud - Assassin Feature
At-Will (1/turn) * Shadow
Free Action - Close burst 10
Target: One enemy you can sense in the burst
Effect: You increment the number of your shrouds on the target by +1, up to a maximum of 4. These shrouds last until you use this power against another enemy or until the end of the encounter.
When you make an attack against the target, before its attack roll, if any, you can choose to invoke all of your shrouds on the target. If you do, your shrouds vanish, each dealing 1d6 extra damage, +1 extra damage for each shroud on the target beyond the first. Reduce the number of shrouds by 1 before dealing this extra damage on a miss.
Creatures you're hidden from aren't aware of your shrouds.
If the target is reduced to 0 hit points or less, you can transfer all but one shroud from it to another enemy you can see within this power's close burst as a free action.
Level 11: 2d6 damage per shroud + 2 damage per shroud beyond the first
Level 21: 3d6 damage per shroud + 3 damage per shroud beyond the first
Special: You can only use this power on your turn.
- Improves the damage output of the striker feature to be competitive and scale competitively.
- Rewards assassins who take the risk of stacking multiple shrouds proportionately.
- Allows the Assassin to compound his shrouds while being stealthy without paying a feat tax (Hidden Insight).
- Helps mitigate shroud wastage on badly damaged or prematurely killed enemies.
Deathstroke: (New Mechanic)
Deathstroke X: When you drop a non-minion creature to 0 hit points or fewer while invoking X or more shrouds with an attack (referred to as a Deathstroke attack) you gain an additional described benefit.
You gain the Shade Form and Shadowcast powers.
Shade Form - Assassin Feature
You transform yourself into a being of shadow, becoming more difficult to notice and to harm.
Encounter * Shadow
Minor Action - Personal
Effect: You assume a shadowy form that lasts until you resolve an attack or until the end of your next turn. While Shade Form is active you gain the following characteristics:
- You are insubstantial.
- You gain vulnerable 5 radiant and resist necrotic equal to 5 + half your level.
- You don't take any penalties for squeezing, and can squeeze to tiny size.
- You can make Stealth checks to become hidden if you have any cover or concealment.
- You can use cover granted by your allies both to become hidden and to remain hidden.
Sustain Minor: The form persists until the end of your next turn.
Deathstroke 4: You gain 1 additional use of this power until the end of your next short or extended rest.
Shadowcast - Assassin Feature
From hidden obscurity, you cast forth a shadowy duplicate to deliver an attack.
Requirement: You must expend a use of your Shade Form power to use this power.
At-Will* Conjuration, Shadow
Minor Action - Close burst 20
Effect: You conjure a shadowy duplicate in an unoccupied square within the burst that lasts until the end of your next turn. It has the following properties:
- 1 hit point, and it can be attacked and is immune to your own attacks if you chose. A missed attack never damages the duplicate.
- You can sense and make attacks as if you were in the duplicate's space as well as your own.
- Resist necrotic equal to 5 + half your level.
- When you take a move action, you can also move the duplicate your speed.
- The duplicate counts as a creature and ally for the purposes of flanking and Shadow Step.
- You can dismiss the duplicate as a free action.
Sustain Move: The duplicate persists until the end of your next turn.
Special: If you make an attack through your duplicate and you were hidden during the attack, you can make a Stealth check to remain hidden after the attack.
- Letting you use Shadowcast in place of Shade Form improves the effective in-combat impact of Shade Form in a thematic way which was lacking due to its exceptionally short 'real' or practical duration (dispelled the moment you attack).
- Rewards the shroud invocation/stacking mechanic, and can help soften the blow of wasted shrouds if the target is badly damaged by the time they are invoked.
- Improves Shade Form to be on par with the superior Black Flame Form alternate class feature power.
Shadow Step - Assassin Feature
You vanish into the shadow energy around one creature and then step out of it near another creature.
At-Will * Shadow, Teleportation
Move Action - Personal
Requirement: You must be adjacent to a creature.
Effect: You teleport 3 squares a square adjacent to another creature.
Level 11: Teleport 4 squares.
Level 21: Teleport 5 squares.
Deathstroke 4: You can use this power once this turn without meeting its requirements as a minor or immediate action, teleporting up to 1 extra square for each shroud you invoked with your Deathstroke attack. You can teleport in or adjacent to a square of dim light or darkness in this way.
- Thematic change that allows the assassin to exploit the darkness and environment, and quickly reposition and engage the next target, or retreat into the shadows after a successful assassination.
- Rewards the shroud invocation/stacking mechanic, and can help soften the blow of wasted shrouds if the target is badly damaged by the time they can be invoked.
Overall not as necessary as the others, but there are definitely solid reasons for its implementation.