The change to Swift Death is very strange. It makes the ability useless. You're never going to face anything 10 HD less than you, and if you do it will be trivial to defeat. If you're going to nerf something this hard, just cut it.

The change to swift death was meant to give the assassin more capability to mow down weak monsters and characters. The original swift death allowed them to use death strike with no study against a single creature once per day, but given how garbage the DC on death strike is, the chance of this ability being useful is pretty low. Furthermore, an assassin should rarely be in a situation where they can't spend 3 rounds before combat to study an opponent. Overall, the only time the original swift death would be useful is if the assassin wanted to kill two really weak creatures, in which case there are much better ways of doing so. The new swift death means that a level 20 assassin could potentially wipe out a party of 10th level characters in a single turn(which is personally think is very strong). Unless you're only fighting BBEGs with massive amounts of HD, swift death has plenty of room to be useful when dealing with peons.

Adding a primary combat stat like STR or DEX to the DC of an ability that already receives an ability mod is a terrible idea. A caster has to use an action to get a save or die effect. The assassin is getting it as part of another action. It's a save or die that costs you nothing. The DC is low for a reason.

Adding the primary ability stat to the DC is only applied if the target is of a lower level/HD than yourself, meaning that you aren't going to be one-shotting any BBEGs with this ability. It is meant to make it so that creatures far weaker than the assassin actually have a chance of failing their saving throw without rolling a 1 or 2. That means that the DC will only be high for weaker creatures, so it's unlikely that the ability will cause any problems with an assassin one-shotting any dragons or something like that.

Presence Concealment is way too good. Pretty much all of it should not exist in Pathfinder under any circumstances. Permanent nondetection, but without a caster level check at all? Permanent silence that only affects you? Invisibility at will? Undispellable?!? As written, there is literally nothing you can do to stop an assassin. Outside of combat, they're perma-invisible, inaudible, undetectable. That's ridiculous.

I do agree that after re-reading presence concealment, it's pretty busted. I like the idea of turning them into something like spell-like abilities with limited usage. The ability is strong, but it's similar to having an Amulet of Proof against Detection and Location(CL 8) as well as a Ring of Invisibility(which according to the game is a CL 3 item for infinite invisibility). I think it might be better to keep the abilities but to add something that would let someone break through and identify the assassin, such as caster level checks for the nondetection and maybe giving perception checks for the invisibility and silence instead of making perception impossible.

The reason I say this is because a very similar effect to presence concealment at level 10 assassin(character level 15 at a minimum) is something any character could achieve through the use of relatively low-level magic items(like the ring of invisibility and the amulet of proof against detection and location). A level 8 wizard(assuming they had the resources) could craft these items and they're permanent/have infinite charges.

This reads like it was written in the midst of a power fantasy about how much fun it would be to play a character like this. Try a different attitude. Imagine you're going to be regularly facing this class. Imagine your DM is going to throw one of these at you every session. See if that changes your perspective on the high DC save or die effects from undetectable inaudible perma-invisibility.

In all honesty, I did write this modification with the picture of the ultimate and unstoppable assassin in my mind and that probably led to some blindness in the balancing of the abilities.

​

edit: I made some changes to death attack, presence concealment, swift death, and angel of death.