But how, I hear you ask?Edit
Well, remember when Truth was killed, and his head spun around thirteen times and flew up into the air? No? Well, maybe you need to watch it again. But anyway, the point is, when Arby went up to kill Truth, Truth started raging and saying how he was 'Truuuth! The Voooiiiice of the Covenant!'. However, what you don't know is that when he said 'Truuuuth!', his head spun around and flew up into the air.It smashed though the roof of the Citadel, smashed through the new Halo, went through the portal, smashed through Cairo Station and ambushed Hood whilst he was drinking his Shiraz wine. The head bit off Hood's Nose and then flew back down again.
It smashed back through the floor of the Station, went back through the Portal, smashed though the other side of the Halo, back through the Citadel and back onto Truth's body. However, it was the wrong way round so Arby took it off and switched it back.After Truth's death, Johnny took the nose from him. However, it escaped and grew into a body... and that body WAS in fact Voldy-Mort.
Weird, huh? Well, if you think about it, it's not really that weird at all. The nose itself is massive, certainly big enough to grow into a humanoid. However, it also explains Voldy-Mort's insanity. Why else would he be obsessed with noses?