Hey! Sorry it took me awhile to get to this; I was updating my laptop and my phone’s shit when it comes to typing on it. But I’m more than happy to answer your question now. 🙂
The Host was originally known as the Author (seen above). As the Author, he appeared as the antagonist in a two-part video with sketch comedy duo Cyndago: Danger in Fiction. A third part was intended, allegedly featuring the Author’s evolution into the Host, but it was scrapped as the teaser (in gif form below) was received poorly at the time.
As the Host, he briefly appears in Markiplier TV. His trenchcoat can also be seen in Markiplier Makes: Fan Fiction, and the thumbnail to Tears of Blood is believed to allude to the Host’s eye injuries. The fandom, for the most part, is currently hoping for a video that will finally bridge the gap between his former life (as the Author) and his current identity (as the Host).
So yeah, to sum up: Danger in Fiction has the Host in his previous incarnation as the Author, and Markiplier TV has the Host in his current form in a brief cameo at the boardroom table. These two sketches are his only actual appearances so far.
Very little is known about him for certain, which is why the fandom is on pins and needles now.