OK...here is what you have created so far.
Lvl 20 Assassin. This math assumes a no special power +3 longbow and no spells, feats, or anything other than the rules presented AND the best poison from the basic rules (since use of poison is inherent in the class design.
Assassin makes a Stealth check at +17 to hide with Advantage (assuming they shrouded the target when sneaking up on them or when the got withing shroud range. I will assume for the math below they succeed at their check and the target is unaware of them at the beginning of battle.
Start of Battle
Initiative is rolled. Regardless of scores only the Assassin gets to go in the surprise round.
Assassins Turn
1. Place 10 shrouds on the target (free)
2. Poison weapon for free (as part of Attack action)
3. Attack roll is +14 to hit with Advantage.
If target is normally hit then Assassin invokes all their shrouds base damage =
1d8 (bow)
+8 (stat and magic)
+10d10 (shrouds)
+11 (class bonus)
+12d6/6d6 (poison DC 19 CON for 1/2)
(1d8+10d10+12d6+19)X2 (vulnerable because shrouded)
(4.5+55+42+19)X2
***************************************
241 HP of damage on a regular hit
+DC19 or unconscious and 1d4+11 damage at start of next turn
+ instant dead if <25 HP
444 HP of damage on a critical hit (18-20 range)
+DC19 or unconscious and 1d4+11 damage at start of next turn
- instant dead if <25 HP
- extra attack for an additional 241/444 damage if its a hit/crit.