@AMetroid Maybe that's it? But I know that when he first died, he had the same item on him on the battlefield, so if it was just an item to help with the initial summoning/contract I don't remember why each individual soldier would've gotten one, instead of just passing one between the soldiers to form the contracts.
@Salfaro Well there was no way to know when a contract would be formed. HERE is the relevant page. It makes them "smell" good so "they" will come wishing to make a contract. So with no way of knowing how to make that happen it only makes sense to increase the number of opportunities and hope for a few hits. Only 4 out of 100(?) managed (with the rest dying?), so 'passing it around' wouldn't have really worked.
@AMetroid That page doesn't really deal with what I'm talking about, because the contract portion I'm talking about is solely in his past life which all happens before that page (the Chapter 15 flashback: https://mangadex.org/chapter/14488/13 ). You see the moment that he forms his contract in the initial timeline via the Blood, alongside another soldier, and it's in a very controlled situation in a tent. We also see him in chapter 1, dead on the battlefield with the Blood around his neck. If the Blood was only necessary to form the initial contract, there would have been no reason to let him take it out of the tent after making the contract, and they were apparently pretty valuable/hard to develop so they would want to keep as many as they could.
Also in regards to that page, in the new timeline she'd already come out on her own in chapter 14, before he even had a Blood (which is why he thought to ask for one to be made), so if it was only needed for the initial contract there would've been no reason for him to ask for a Blood to be made in this timeline, since his contract was what dragged her into this timeline anyway. So it seems like even if the Blood is needed for the initial contract, it's still needed in order to forcibly call the spirit too. Basically like bribing a cat with treats: They're already your pet, but it's easier to make them do things with a lure.