Eren made the Holy Priestess an offer. In exchange for the information on the completion of the dagger, she could receive her life.
It was still unclear if Eren actually meant what he said, or simply he was simply trying to scam her even in the last few days of her life.
He could use soul search on the Holy Priestess to read her memories. However, her soul had already been damaged, becoming more unstable in the process of possession.
She didn't just lose her lifespan but her soul was also damaged. If he tried searching her soul in this statement, he wasn't even sure if her soul was going to survive until he finished looking for the information that he wanted.
If the Holy Priestess could help him out of her own volition, then the risks were minimised.
"Without my help, you will die in two days. If you help me complete this dagger, I can help you live longer," Eren told the Holy Priestess, whose eyes revealed a little interest.
