In order to easily kill Tsorig's Invader form just open the gate blocking the path to the destructable bridge first.Tsorig can be stunlocked by most weapons with poise damage.If you have not gained The Knight Slayer's Ring from killing his Invader Phantom, killing his NPC will net the item along with the Fume Greatsword and Black Iron Greatshield. There's a pathway leading down to where Tsorig is waiting. After the basilisk room, instead of going up the stairs to the bridge, look left instead. He is found under the bridge over the lava pool which leads to the ballista. You can find him in Smouldering Lake (as a hostile NPC, not a phantom), near the Demon Ruins/Old Kings Antechamber bonfires.Tsorig will invade upon you entering this hallway, attempting to trap you between the doorway and a wall of iron bars. The quickest way to get the invasion is to follow the skeleton boulder from the bonfire, and take the doorway just to the right of where the boulder hits the wall. Note that Tsorig can be summoned even after you kill him in person near the lava pond, assuming you killed his dark spirit form in the Catacombs. The sign is in a corner of the Demon Ruins bonfire room. Can be summoned for the Old Demon King boss fight, only after he has been killed as an invader (killing his NPC form will not cause the sign to appear).If you kill him or he kills you, you obtain the gesture: ' My thanks!' which is 'War Cry' from Dark Souls 2.However, if he is still alive, he can still be killed after Wolnir is dead. Wolnir must still be alive for the invasion to occur. Invades you shortly after the first bonfire within the catacombs, near the bridge approaching High Lord Wolnir.Knight Slayer Tsorig is a NPC in Dark Souls 3

Dark souls 3 war cry