When you use a humanity to reverse your hallowing in order to summon a friend, you are also then opening yourself up to be invaded by other players.

If you die while an invader is in your world, they gain your lost humanity as well as some souls (not your souls).

There exists a covenant, or a faction, that actually punishes those invaders. Their sole purpose is to invade your invaders and avenge you, and others who have died to them. They are the Blade of the Darkmoon.

But the Blade of the Darkmoon cannot carry out their duty without help from you, the victim.

Indicting players who have invaded your world and killed you is the only way the Darkmoon Blade can invade your killers. In order for you to indict, all you need to do is buy ONE (1) indictment from Oswald of Carim, the Bishop whom you meet after ringing the first bell of awakening after the gargoyle boss fight.
Here he is in all his glory.

In order for him to appear, you have to beat the gargoyles in the Undead Parish, continue forward into the bell tower, climb the ladders, and ring the first bell of awakening. Once that has been done, don't homeward bone - instead, climb back down the ladders to the base of the tower, and you will find him just as he is in the above picture.
Talk to him and purchase a single indictment from him. One indictment is all you need to indict as many players as you wish. You can now add your killer's name to the list of sinners who will face retribution from the Blade of the Darkmoon. Whenever you die while an invader is in your world, a prompt will appear asking if you want to indict the invader or not. I think we all know the answer.

I created this guide to hopefully spread the word about this aspect of the game, because I noticed how hard it is for BotD players to get any invasions. PVP is a big aspect of Dark Souls, whether people like it or not.

Invading sinners in the original Dark Souls: Prepare to Die Edition PC version was one of my favorite passtimes. Something about invading and killing the bastards who murdered other innocent players was very fulfilling.

Here's my all-time favorite moment, right after having killed a sinner and his white phantom friend:
The sword in question being the almighty Murakumo.

Thanks for reading!