The Senior Partners (also known as The Wolf, The Ram, and The Hart) were the chief antagonists of virtually all of the Angel series - however they were never seen and instead influenced the world via their countless minions - who formed the Wolfram & Hart institute as a means to make sure the Apocalypse went to plan, as well as this powerful society the Senior Partners had influence over the elite demon-society known as the Circle Of The Black Thorn: who were tasked with the job of ensuring "man's inhumanity to man" steadily increased and thus keep the gears of Armageddon rolling.
Due to Angel's determination the Senior Partners were delivered a terrible blow near the end of the televised series when he successfully infiltrated the Circle Of The Black Thorn as well as killing the current "Child" of the Senior Partners known as Marcus Hamilton.
In retaliation to these actions the Senior Partners destroyed the LA department of Wolfram & Hart before dragging the city into a living hell in an apocalyptic event known as the Fall Of Los Angeles - during this time Angel and his crew fought to free LA from many Demon Lords but never engaged the Senior Partners directly: indeed it is heavily suggested throughout the series that the Senior Partners simply can NOT be faced directly - such is there immense power.
However, one of the Senior Partners does appear in the second season, and is killed by Angel; the other Senior Partners never directly interact with Angel Investigations.
Somewhat ironically despite their near godlike powers in the modern age the Senior Partners were once considered minor demons, no more powerful than vampires, however after leaving the Earth Plane it seems the demons managed to amass power far exceeding that which they had in the past.