MMOexp: If you're an avid (or extremely aggressive) Elden Ring player, you may have killed a handful of NPCs at random, and not even realizing they were in fact crucial to the quest, or that they were holding onto an crucial item. But you could be lucky: A handful of Elden Ring Runes slain NPCs aren't dead forever or perhaps forever mad at you after a resurrected. In order to clean the slate all you need to do is go at the Church of Vows and say "sorry for my mistake" for killing the NPCs. It's a little more complex than the previous. However, it's not much.
You can visit the crumbling church located in Eastern Liurnia, just west of the Ainsel River Well (or slightly south of the Raya Lucaria Crystal Tunnel). The first thing you'll see is the gigantic tortoise Miriel and is easily identified by her cute pope hat. However, you don't really need to speak to Miriel to meet her. What you really want is the Lady Rennala at the back. The offering of a consumable known as the "Celestial Dew" to the statue of the fairy lady will bring about the revival of many of NPCs in the game, and the revived NPCs will forget any animosity they may have been able to show towards you.
The problem is the fact that not every NPCs are able to be revived by this method. Since the game is so new there are many discoveries being made, and knowledge is in a state of change, and it's not clear why certain NPCs are able to be revived, while others aren't. As long as the community doesn't compile more precise information it's best to be optimistic when you're trying to revive someone. (That being said, making the offer does appear to erase the living NPCs who are angry at you even if they're not listed on the "resurrectible" list.)
In any event the Celestial Dew that required to complete the ritual can be obtained through various ways. There's a vendor who sells it in the vicinity of the Ainsel River, or Pidia at the Caria Manor in northern Liurnia. It is also possible to obtain the dew in the traditional RPG method: by looting it from corpses. There are a few of them if you begin trawling through certain corpses found in The Uhl Palace Ruins, Nokron, Night's Sacred Ground, Nokstella, and Raya Lucaria Academy. It's the smallest penalty you could get to take a demon's life isn't it? You monster.
In terms of NPCs They're more easily to locate now that the most recent Elden Ring update also added useful NPC symbols to maps. If you're in an eerie mood and are looking for possible victims, take a look at your map. (Again it's true that you're an abomination.) Some NPCs are not rehabilitable take note. Therefore, think very cautiously before you attempt to kill one of buy Elden Ring Runes them.