The first time you meet Seath in Duke's Archives, you are forced to lose. It isn't until you get to the Crystal Cavern that you can fight and kill Seath. It is strongly advised to have some curse resistant equipment with you (Cursebite Ring should be sufficient), as one of Seath's attacks can curse you. It also might be better to not equip heavy armor, as you will need the mobility. Before you even begin to do damage to Seath, you will need to destroy the glowing crystal shard thing behind him; if you don't, then Seath will regenerate any health that is lost (you need to do this every time you die). Seath doesn't have many attacks, but he does have some nasty ones. The main one you should be watching for is his breath attack, which spawns crystals on the ground and if you come into contact with them, you will take damage and your curse meter will rise (you can't block the breath attack). It has two variants of this attack, either spawning at the player's point or from left to right in front of Seath. Seath can also lash its small tails around, as well as his big tail, which isn't visible from the front, so you have to be careful when you're behind him. He can also use his left arm to swipe in front of him, as well as use an explosion in his close vicinity. You will want to try and keep Seath in the center of the room, so you won't get trapped, which might happen with his tails. There is a blind spot on Seath's left tail, so that will be where you want to attack from to damage him. Don't get too close, as he'll use his tail to hit you and he might spread crystals all around him. Note that you need to hit the tip of the large tail (the one directly behind him) in order to cut it off.