Burich can be found in Dragomir's Tomb in Baldur's Gate II: Enhanced Edition.

To get to Burich you need to operate the tomb (use it) then crawl in. You may need to take two or more characters with you to defeat him but he gives you the rod you need to complete the stage. You need to take the rod back to the entrance room and use it in the wall where the light is highlighting.

You can also talk him round to giving you the rod without a fight, by questioning his orders then promising to defeat Dragomir. May require high charisma.