The Arkhalis is a unique sword obtained only during Hardmode and in Expert or Master mode. Dropped as a part of the Arkhalis Developer Set, this sword is very rare but is always nice to find. It’s unique attack makes it difficult but rewarding to use despite its limited viability later in Hardmode.
Getting the Arkhalis is difficult, as it will only drop as a part of a Developer Set. Developer Sets are special vanity sets which can be obtained as loot from opened Treasure Bags in Hardmode. This means the sword is only obtainable in Expert or Master mode. Developer Sets are made for the developers of Terraria and are often named after them. This set, as well as the Arkhalis sword, are named after former Quality Director at Re-Logic, Arkhayla.
The easiest way to obtain the Arkhalis, and by extension, any Developer Set, is to simply open a lot of Treasure Bags in Hardmode. Unfortunately, this doesn’t mean you can just kill King Slime a bunch to get it. Developer sets have a five percent chance to drop from Treasure Bags of Hardmode bosses only (excluding the Queen Slime for some reason). So the best bet is to farm one of the Mech Bosses for it, which is far more difficult than farming King Slime. Fortunately, after you beat one Mech, all three become vastly easier and you should be able to farm Treasure Bags pretty quickly after you manage to kill at least one.
The Arkhalis is based on the Crissaegrim from the game ‘Castlevania: Symphony of the Night’ as it is Arkhayla’s favorite weapon from the game. When used it doesn’t swing like a normal sword, instead it releases a flurry of blurred slashes in the direction of the cursor which inflicts damage very quickly. Its base damage may seem low but its speed is what makes it exceptional.
The weapon’s low range makes it difficult to get used to but once you do you’ll find it's very useful. First, its fast, piercing attacks means it's very good against worm enemies and its incredible speed means it's effective at destroying projectiles like Chaos Balls from Goblin Summoners and Fire Orbs from Imps. It also emits a few light particles in the direction of its swing, making it useful for seeing through cave walls.
The Arkhalis is a Hardmode sword found only in Expert or Master mode Treasure Bags. It's incredibly fast but short ranged, meaning it's hard to use but still an effective weapon. Obtaining it can be difficult though due to the rarity and difficulty of obtaining Developer Sets.
The Terragrim is another sword that behaves exactly the same as the Arkhalis. It has the same attack style and only slightly less damage. Its name comes directly from the Crissaegrim this time and it can be found pre-Hardmode in sword shrines, similar to how the Enchanted Sword is found.
The tooltip is a reference to ‘Castlevania: Symphony of the Night’ again. A Schmoo is an enemy which drops the Crissaegrim in that game, so with the Arkhalis being based on that weapon, the reference is clear.
Interestingly, both the Arkhalis and the Terragrim are the only swords that cannot receive the Legendary modifier. Instead, the Godly or Demonic modifiers are the best for maximum damage output.