Is it just me, or does ninja feel like...

[Raikai]

Assassination was originally a move that could only be used with an enemy was at low HP, the removed all those kind of skills from the game and so Assassinate was removed along with them. It was later added back as an expansion capstone ability to add extra damage, but was easier to just use the same icon and animation rather than create a new one.

Still, doesn't make any sense to make it tied to Dream Within a Dream... Seems just for the sake of it.

there's not much need for an extra benefit on Doton, especially since tanks don't tend to like to hold mobs in the DOT circle anyway (and as one of only two of that kind of move left in the game

That's the point of the "upgraded" version via Kassatsu. It would not be a land AoE, but a stronger poison DoT on everything.