Alaric has left the building (or may be sleeping in a little dusty corner of his mind) whilst Big Bad Klaus is spiritually hitch-hiking in his body.
So, Alaric's body and Klaus' mind (and accent)
(It helps to watch Supernatural which has a rich history of demonic and angelic possession, vampiric possession is easily hand-waved 
)
As to why/how/is this a new thing? I have no idea. I'm hoping the next episode will explain more!