20what they are actually for
Very few players clear content with these as a primary weapon. The point is the drain. Swap to them when your health dips, spray into whatever is close for a second or two, swap back. Against a crowd the healing outruns most incoming damage and fights that were killing you stop being close. That keeps them in the hotbar long after their damage stops being relevant.
how the throw behaves
Each use puts out a fan of knives across a wide arc, and one of them always travels exactly where the cursor is, so hitting a single target is more reliable than the spread looks. The knives ignore gravity but they do not go far. After a short flight they begin spinning and fade out, losing damage and knockback as they go, which makes this a close to mid range weapon whatever its listed velocity implies. Melee speed pushes the knives out faster without making you throw more of them.
where the heal breaks down
- The return is a percentage rounded down, so knives that only chip at a high defense target heal you for nothing at all.
- Health regained per second from any lifesteal source is capped, so past a point more knives connecting does not mean more healing.
- Moon Lord applies Moon Bite, which switches the effect off completely. Plan that fight as though the knives were an ordinary weapon.
- A knife reflected back at you does not heal anyone.
- A Flask of Ichor raises the damage each knife deals, and the heal scales with it.
the Crawltipede exception
A Crawltipede tail takes multiplied damage from everything, and the lifesteal multiplies right along with it, so one volley into an exposed tail refills an enormous chunk of health during the Lunar events. The per second cap stretches the gaps between heals, but nothing else available in that fight restores health that fast.
getting them, and the modifier
One chest in the Dungeon holds these and nothing else in the game does. It needs a Crimson Key, which only drops from hardmode Crimson enemies, and the Crimson Chest itself stays locked until Plantera is dead. In a Corruption world you have to seed a Crimson biome and let it spread before a key can ever appear, so start that early rather than after Plantera. Godly and Demonic come out equal on average, so take whichever you roll first.