Ilend Vonius is an Imperial Guard, and one of two guards found alive in the quest "Breaking the Siege of Kvatch." After killing the surrounding Daedra in the area, the Hero can choose to let him help them, or to send him back to Savlian Matius.
If he survives, he will assist them in the quest "The Battle for Castle Kvatch," and if he survives all Kvatch related quests, he will wander around Castle Kvatch, constantly thanking the Hero for saving his life.
When first talking to Ilend, you will find his disposition of the Hero is maxed out at 100.
The Hero learns they have to go into the Oblivion gate and shut it down in order to rescue Martin and the others.