Death Defying Grit

I have no real idea If this would actually be that useful I haven't even played a full DND Campaign nor ever DM'd and don't got anyone to play with anyway and I'm making a homebrew dm from scratch based off a goddamn dream I do have 8-9 pages full of concepts and ideas and many more to come for it though anyway this is based of the death defying/tricky escape feat/trait/Strength from Middle Earth:Shadow of War/Shadow of Mordor that happens when a orc just rage baits you before escaping automatically or healing and saying "not yet" or whatever I haven't played it in a decent while.

When You Reach 0HP And Fail/Critically Fail your Final Death Save/Any point where a enemy can kill/execute you or is going to kill/execute you as long as the enemy is within Attack Range(up to DM or how well you can argue you can reach/how far you weapon will reach) You Can Instantly Kill them and recover to 5hp or can Brualllize them and recover with 3hp causing Fear in Nearby Creatures that Fail a Wisdom save and causing already fear inflicted creatures to flee/back away from the character(instant kill does not work on Huge nor Gargantuan Enemies Unless They are Below or at 5hp or DM allows it and if a huge or gargantuan sized enemy dies all enemy units gain fear or flee or back away what enemys react like are also up to DM also gives any allys that can view it Hope:Boosted By 5 temp hp and recovers from fear/any fear like effect up to the DM/)(if a huge or gargantuan creature dies you survive at 2hp and causes Exhaustion) does not work on fully okay/not critically injured bosses/mini bosses/BBEG (up to Dm if they deem it'd be funny)also obviously can only be used once per 5 long rests/10 short rests/up to Dm.

Any comments or ideas to help make this not suck would be nice!