Elden Ring PC | Invading Old Knight Istvan in Limgrave, defeat & drops, Volcano Manor quest, on NG

Location: Limgrave, near the Warmaster's Shack

Drops:
2,193 runes
1x Rune Arc
1x Furlcalling Finger Remedy

His red invasion sign will not appear until you're sent on the quest by Volcano Manor.

Old Knight Istvan
"Old Knight Istvan is an NPC in Elden Ring. He can be used as an NPC Summon. His sign is just outside the Demi-Human Chief boss arena of the Coastal Cave as long as he is not killed. He is also one of the assassination targets assigned to you by the Volcano Manor. You can find the summon sign to invade his world in Stormhill."
https://eldenring.wiki.fextralife.com/Old+Knight+Istvan