Newcomer is the actual name of the faction largely populated by Tremere clan in Vampire the Masquerade: Bloodlines 2. It refers to the fact that the clan has been kept out of Seattle until recently.
According to the lore of Vampire the Masquerade: Bloodlines 2, the previously somewhat reclusive clan of Tremere were kept away from Seattle for a long time for reasons unexplained in the reveal. It is possible this had something to do with either the blood magic they are synonymous with or their influence which immediately started to grow once the visionary Prince Cross let them back in, which naturally surprised everyone else.
The leader of the Newcomers is Viktor Goga, often called the "Professor", who offers members of his faction virtually boundless reserve of high-quality blood on top of being a member of an organisation that's quickly rising to popularity.
It is likely for this reason that The Baron finds The Professor to be his biggest threat due to the latter's growing influence as influencing others from behind the scenes is The Baron's bread and butter, more or less.
There is a mention of a high price the Tremere had to pay to get back into Seattle but it's not revealed what it was, so chances are that Cross simply wanted to undermine Baron with a peer keen on developing more of their own influence.
As you may have noticed from the header image, it appears that Elif from the gameplay demo that is all over YouTube actually belongs to The Newcomers. As of the time of writing, it is unclear whether she is a high ranking member.