How do you get yosuke's malakh in persona 4?

Answer 1

To get 'Malakh', Yosuke's strongest weapon, first you need to have beaten the game at least once.
On your 'New Game+', a new enemy called 'The Reaper' will randomly appear when you open a normal treasure chest. If you beat him he will randomly drop a weapon. an armor or the 'Omnipotent Orb' (This last one being the harder to get).
I recommend you to be at level 70 or higher since it's one of the hardest enemies to beat and to have Yosuke in your party as well (This will lower the odds of getting it).
The Reaper appears more frequently in the real killer's dungeon and usually on the lower floors of the other dungeons.

Note that on the PSVita version, Persona 4 Golden, the Reaper can even appear during your first playthrough.