it would realy depend on wich sword you use.....nightingale swords arent good because you cant upgrade them but if yu use the dawnbreaker legndary(600+ damage) you would pwn with it......i(if you havent already) join the dark brotherhood and pickbocke this woman who is like the leader and you will get the balde of woe off of her then complete the storyline and you will get another blade of woe. but f your going heavy then get this massive hammer called Volendrung....
You can't Improve it at tier 5 (over level 40 something) Tier 4 has just as much Absorb health, and 5 less Absorb Stamina. However, that's just if you're Crafting, there's really no need with the full set, and you can keep your character level lower without the extra trees. This is important, because CL determines the level of the NPCs, so you can smith for more damage, but you end up needing it, because the enemies are tougher. And besides, Assassin is already a multi-discipline build even without throwing in Illusion, much less Smithy, Enchantment, and no doubt Alchemy to run crafting loops, and Fort Resto glitches. Unless you're playing on Master, you can do fine with just 1H, Archery, Sneak, and LA, if you're any good at Assassination to begin with. With Backstab bonuses, effectively doubling your level for extra damage you don't need is actually counter-productive.
Illusion isn't mandatory, because of the 2 effects you'd Need it for, Muffle is built into the boots, and Invisibility is doable with Shadow Warrior. Until then, you can use Vampire Dust (no need to mix it) the Shadow Stone, and Shadowcloak of Nocturnal the few times you actually need it. Honestly, Sneak is OP enough without it.
Skyrim Perk Calculator - Plan the perks for your Skyrim character before spending them! This is all you NEED. There's some other stuff that would be nice, but you can build off this foundation. Agile Defender(1) because you shouldn't be taking hits unless something went wrong, Stealth(1) because you don't need more than that to go undetected, especially in Muffled boots with such a good Bow. Steady Hand is great, in battle, but when the target has no idea you're there, and may be walking back, and forth on a preset path, you don't even have to lead them that much. Critical Charge to combo with Silent Roll (It helps to have Unhindered, and Wind Walker for this combo, since it's a Sprint+Power Attack) to burst from the shadows before they can react, great Ambush tactic.
You MIGHT want to add Alchemy, since that can do Paralysis, Frenzy, Invisibility, and so much else without ever using Magicka. Also, pick up Linwe's Hood, you have to do the TG quest anyway, and Fortify Archery is so much better than Illusion, even If you're using it. (More Damage vs less Magicka should be a no brainer every time.)
Destroy the Dark Brotherhood, for Nocturnal. One cannot serve two Masters without pissing them both off (Okay, you can in this game, but for Role Playing) and you can get the Blade of Woe without grinding up Pickpocket to Perfect Touch for 1 item. I didn't mention them, because this is a Nightingale Build, but the Shrouded Gloves really help out with the Nightingale Blade (tier 5) Even without Smithing, and Assassin's Blade, 12x Daedric damage with Armsman(5) + the Absorb Health is plenty for anything you can Backstab on Adept difficulty. For everything else (Dragons on the wing) you have a very nice Bow.
Volendrung is just bad advice. It has nothing to do with the Nightingales, (other than being in the Rift) is 2 Handed, so there's NO WAY to increase the backstab multiplier, and you have to kill Giants for Malacath, which is pretty much the opposite of the Nightingale's charter. I normally don't judge people's advice purely on the basis of retarded speling, and a complete, and utter lack of grammar, but it's justifiable when it's so downright crackheadded.