How does summoning players for bosses affect boss stats?

Question

When fighting bosses, it's fairly obvious that summoning four other players will increase the bosses' total health by quite a bit, but what exactly is happening here?

• Does each summoned player increase a boss' health by a certain amount?
• Do NPC summons have the same effect as player summons?
• Besides health, are any other boss stats affected by having summoned help?

Specifically, is there a linear increase in health between summoning one and three players? And when I summon another player and they get killed right after the first attack, how much more work did I just sign myself up for?

Answer 1

Each summoned player, or NPC will fractionally increase the health of the boss. I don't know offhand by how much for each summon. You can have up to three summons. Basically, it's there to make sure you can't cheese the bosses.

A great wiki to use for information about the lore, mechanics, etc. of Dark Souls is the Fextralife wiki. I highly recommend checking it out. Edit: Boss damage, resistances, and weaknesses do not change.

Edit 2: I did some looking around on forums and Reddit, and the general consensus is between 10-20% more health per summon. In Dark Souls 1, health was increased 50% per summon, and in Dark Souls 2, boss defences increased.

Edit 3: To answer the second part of your question, when your summon(s) enter the boss room, their health is automatically increased, just like the enemies outside of the boss rooms. The effect stays until either you die, or defeat the boss, no matter how many summons die.