Rtas 'Vadum in crashed Flood ship
From Halopedia, the Halo wiki
When you jump down the hole into the Indulgence of Conviction's crash site, there will be several dead Elite Ultras scattered around. Sometimes, one or more will have Rtas 'Vadum's face, in which two mandibles are missing. You can get it infected, too, if you push it down the hole and shoot some Flood blisters to release pod infectors. When the Elite turns into a Flood Combat Form, his armor will still change to default blue, like all infected Elites, when the body is destroyed.