The slave contract was formed without any glitches.
It offered Eren complete security from Lagartha. She couldn't lie to him or deceive him without him not knowing about it. She couldn't willingly, subconsciously, or follow someone's orders to hurt him.
Eren could control Lagartha's actions to a large extent. But he wasn't able to dictate her will or her emotions. However, the control he had over her was enough to trust her.
Only after confirming his connection with Lagartha did Eren take a breath of relief. He didn't show it. But he was on the edge dealing with seven Spiders all by himself as well. He didn't think he was in mortal peril. But he could never be too sure about what his enemies could have in store for him.
"I hope you don't ask of me something I find difficult to provide."
Lagartha said to Eren while gulping down another vial. She was really exhausted fighting with Eren. Mentally more so than physically.
