Bloodworms act identically to Truffle Worms, slowly crawling along the floor and burrowing away from the player when they get too close to it. Also like Truffle Worms, Bloodworms cannot be used in normal fishing; instead, they are used to summon The Old Duke while fishing in the Sulphurous Sea.
Bloodworms can also be purchased from the Sea King for 2 after The Old Duke has been defeated.
Bloodworms only spawn close to the bottom of the Sulphurous Sea, requring the player to dive deep underwater to find and catch them. Due to the depth they spawn, it's also possible for them to spawn near the top of the Abyss.
Bloodworms will not spawn if the Acid Rain event is currently ongoing.
Bloodworms will always be consumed upon summoning The Old Duke, regardless of any items the player has that reduce bait consumption.
Bloodworms can be found and caught in Pre-Hardmode, despite the fact that The Old Duke is supposed to be fought after defeating Polterghast, a post-Moon Lord boss.
The spawn rate is increased the lower the amount of other Bloodworms alive in the world:
If there are no Bloodworms, spawn rate is boosted by 400%.
If there is one Bloodworm, spawn rate is boosted by 256%.
If there are two Bloodworms, spawn rate is boosted by 144%.
If there are three Bloodworms, spawn rate is boosted by 64%.
If there are four Bloodworms, spawn rate is boosted by 16%.
Spawn rate will not be boosted if five or more bloodworms are alive.
The bait power's usage of the number 4 is a reference to superstition held in some Asian countries, such as China and Japan, about the mentioned number, due to it being a homonym for "death".
If the player attempts to summon The Old Duke using a multi-lined hook, such as the Early Bloom Rod, it will lead to multiple Old Dukes spawning in at once.
This is an homage to an oversight in Vanilla Terraria, where multiple Duke Fishrons can be summoned using a fishing exploit.
Similarly to the Truffle Worm, the Fisherman's Pocket Guide (and its combined accessories) will display a player's fishing power simply as "Warning!", due to the fact that The Old Duke will spawn when using this as bait in the Sulphurous Sea biome and no fish will be caught when not in the Sulphurous Sea biome.
In real-world bodies of water, the presence of bloodworms is an indicator of pollution, therefore it makes sense for this kind of creature to be present in the Sulphurous Sea, which is highly acidic and polluted.
Previously, the item's bait power was 69420%, as 69 and 420 are both numbers which are used in memes.