After Knife and Death finishes resolving, they return from whence they came, to thine sideboard or collection
If it leaves your hand before then, it wont enter the battlefield at all
However, that product is aimed more at beginners who are new to Lorcana, so it wont be quite as exciting for those of us who are already well accustomed to the nuances of the game
As far as strategy goes I would recommend using the other formes of Deoxys in this deck so you can utilize its Poke-Power and I would also try and wait quite long to use this card so you can make Energy Crush more effective, the longer you wait the more energy cards your opponent will have attached to their Pokemon
If you know, or suspect, your opponent is playing Tron, then feel free to keep it, but in most Games 1 you will have a better chance if you find a different opening hand
98 non-foil cards