As Blackrock Depths is a very complicated instance, some players have difficulty in finding their way around. So here is a guide on how to find Loregrain.
From the start of the instance, head towards the arena (through either of the two north-eastern tunnels) and complete the mini-event.
Go through the gate that is opened after killing the boss, head up the ramp, turn 180 degrees right, and then go up the stairs and onto the arena second floor.
Go around the pit, and hang a right at the T-intersection. Kill the trash and the Pyromancer will be in front of you.
Loregrain comes with two Twilight Emissaryadds, but is a very easy mini-boss. He drops Searing Totems and casts Flame Shock occasionally. He spams Molten Blast which can deal lots of damage in a short time , but generally is a pushover. He can be sheeped or MCed while the adds are dealt with.
Most classes can solo Loregrain, though few will survive long after looting him. All non-stealth classes will need to use an invisibility potion after the arena to get to him.