Keepers of the Flame: Assassin Rework Overview
Overview
Guide
More Build Archetypes, Less One Dimensional
It appears that the main goal of this rework was to enable the Assassin on a more diverse array of build archetypes.

Toxic Delivery & Infused Toxins
The new Toxic Delivery notable pulls some of the highlights from the previous Critical-Poison focused branch, providing a damage boost, as well as scaling Poison Duration and granting a much needed source of recovery.

But the Assassin already boasted a mastery over Critical-Poisons in older patches – what it has gained in 3.27 is the ability to specialise into Poison application via Elemental Damage using the new Infused Toxins notable.

This node provides a powerful trait that is only unlocked via other means by sacrificing gear slots, such as the use of a
Pneumatic Dagger base. With Infused Toxins, the Assassin will be able to pull of Elemental-based Poison builds without the gear strain, enabling new combinations of mechanics and affording this Ascendancy more freedom in this archetype.
Shadowed Blood
Another new notable on the reworked Assassin is Shadowed Blood, a defensively-oriented node that will enable some potentially very strong offensive options. Being Unaffected by Damaging Ailments is quite a strong benefit, making Bleeding, Poison and Ignite still applicable to you, but otherwise rendering them useless as they no longer deal their damage. You can use this to your advantage with strategies such as self-Ignite using
Mokou's Embrace.

Meanwhile the potential of this notable is elevated due to the existence of
Dissolution of the Flesh, a jewel that makes Life loss from taking damage be reserved for a period of time instead. One of the major aggravating factors for the playstyle of this jewel is incoming damage over time, so being able to exclude Damaging Ailments entirely is quite the benefit. The recoup provided here is also quite useful alongside
Petrified Blood, another upside to the typical Dissolution setup.
Mystical Infusion

One of the new 4-point notables is Mystical Infusion, a node that grants a modifier previously only obtainable via use of
Sandstorm Visage, a helmet that carries the heavy downside of preventing Attack Critical Strikes. This new notable will overwrite your base Spell Critical Strike Chance with that of your Main Hand weapon, providing the opportunity for very high base Critical Strike Chance considering some weapons can reach excessively high Critical Strike Chances with good modifiers. Meanwhile Mystic Infusion is most notable for its incredible potential as a basis for Cast On Critical Strike Support builds, using Critical Attacks to trigger Spells that then also deal Critical Damage.
The Rest
Whilst the new Assassin gained the specialisation into Elemental-Poisons and CoC setups, it isn't all upsides. The previously strong Power Charge stacking benefits of the old
Deadly Infusion, which provided Critical Strike Multiplier per Power Charge on top of a small addition to base Critical Strike Chance, is no more. The new Assassin as still retained the extra Maximum Power Charge on the Unstable Infusion notable, which is almost unchanged, but Power Charge stacking specifically has lost some power this patch – no pun intended!
Opportunistic is another notable with a bit of push and pull from the old Assassin to the new. This node previously only cost 2 Ascendancy Passive Points to reach, and for some builds it was worth the low investment for some situationally strong bonuses on top of the useful Reflect immunity. But the new Assassin has this node locked behind another 2-pointer which also has situational stats to boot, so it's unlikely that this 4-pointer is going to be too popular in the future outside of use on builds that are already sinking points into Knife in the Back or For the Jugular. However, it should be noted that in situations where the Less Damage Taken component of Opportunistic is turned on, such as during the Maven's brain-battle phase, this modifier provides incredibly strong damage reduction with a massive buff up to 35% Less this patch, up from the 20% Reduced in older game versions.
In regards to the Knife in the Back or For the Jugular choice notable, it should be noted that the possibility is there for Forbidden Flesh and Flame jewels to provide access to both of these notables at once - one from the Assassin ascendancy choice itself, and the other from a named jewel combo. The precedent already exists on the Scion's Ascendant, the only other ascendancy that offers choice branches - that class is able to obtain two choices from the same branch using Forbidden Flesh and Flame jewels, so the same may be true for Assassin in 3.27.
One of the other new notables on the Assassin in Keepers of the Flame is Deathmarked, a node that provides some automated Assassin's Mark action. This node isn't particularly powerful, mostly due to the fact that a level 30 Assassin's Mark is much better than its level 20 counterpart. However, Deathmarked can be used to solve possible mana cost or socket pressure issues in a pinch.