(Oh god here we go...)
Isaiah moved back before Lawrence dragged him into the darkling mass with him. A small part of him was disappointed he hadn't killed him himself, the larger part was glad and hoped he died screaming. "Wait for me in Hell, wretched knight, we'll finish this then." He struck a few darklings that reached for him, sending them back to dark Hell from whence they came. The rest were too distracted with Lawrence to pay him any mind, ad he took advantage of the fact. 

Isaiah turned to where Lorelai, Irena and Steffan were gathered just in time to see Lorelai charge at Steffan with the remaining sliver her dark blade. "Princess no!" . He was fast enough to reach them in time but not fast enough to put himself between the two in time to block the blade. Instead he thrust his black sword forward in attempt to block Lorelai's attack. He moved on instinct, he was knight sworn to defend his king and he had been tempered and conditioned since his youth to move and defend him from any assailants without so much as a second thought. It was because of this instinct that Isaiah didn't realize he had overshot his blade.