This is the other thing with Assassin as a class- their entire theoretical focus is the quick kill, but to me instant death doesn't make for a great mechanic for a PC class. To keep the Assassin from blowing up every encounter with instant kills at higher levels, you start facing the temptation to introduce immunities- which then turns the Assassin back into a Rogue. How do you balance something like that with the other classes, so that the Assassin doesn't end up either the star of every encounter (when he insta-kills the big bad after a few rounds of doing nothing) or really bored (when he waits too many rounds, and the big bad dies before he can do anything because he was too busy chargin up hz lazers)
The keys is to branch the aspects of his assassination skills into other pillars.
Here are my ideas.
1) Let the assassin craft poisons and explosives so they have something to do while stalking.
2) Make death attack weak but useful when not powered up. Branch the set up into other pillars.
Death Attack: Assassins are natural born killers. They are lethal combatants and are feared for their murderous ways. With a dagger, arrow, or spell, an assassin can end the life of an enemy or target in an instant.
Benefit: Once per round, you can make a Death Attack attempt against someone. If you successfully deal damage to an enemy that round, you make make a. Death Attack check. If the check is successful, the target must make a Constitution saving throw or die instantly. The DC for this saving throw is 11+ your Intelligence modifier. As you level, the DC may increase as well.
The base chance of Death Attack effect is 5%. As you level, your Death Attack chance increases. At no time can your death attack chance exceed 95%.
Cold Focus: Assassin have full control of themselves and an almost supernatural awareness of their bodies and the bodies of others. With this obsessive attention to detail, assassins are deadlier and harder to deal with when their eyes are on you.
Benefit: As an action, you can focus on someone not hidden from you. You have advantage on any contests opposing this individual. You can only focus only on one target at a time per assassin level. Your focus only last as long as the target is not hidden from you.
In addition, your focus increases your Death Attack chance by 10%. You can repeatedly focus on an individual, each time increasing you death attack chance. If you fail a Death Attack attempt or the target becomes hidden from you, your Death Attack chance resets back to the value given by your assassin levels.
3) Make assassins squishy so they cannot KO BBEGs without serious help. Solo assassins can only kill nobodies.