_ _ ( `\/' ) `\ /' |\/| | | |~ | | | | ~| | | | | | ~| |~ | | | | ~| /~~([--])~~\ | | | | | | "OO"
This is a challenging sword to win with. It gives you life after a kill, so it only benefits you when you successfully kill something. In theory it is useful against swarms of small enemies that don't hit hard, but the other swords are also good in that scenario. The fire sword will have you hitting them less often, and if I understand how the summoning sword works, it will deflect damage to you by putting a tank in between you and the enemy.
The sword of life is especially challenging on the Castle's entrance level, because the slower speed of clearing enemies means that more total enemies will spawn, and the magic star projectile will have an easier chance catching up with you.
The game is beatable with this sword, but be prepared for a challenge.