This enemy is usually in Silverlight Hils, inside the Stronghold, on the eastern side of the area. He is a level 80 boss, so it will be very important that you level up your weapons and equipment before going with him.
You can use the Blood Altar to track this enemy's blood trail. Make sure you complete The Hunt quest to get the Altar of Blood blueprint, for which you will need 180 Stones and 10 Blood Essence to build it via the 'Build' menu.
Eliminating the final boss will allow you to become the Vampire Lord.
Now that you know where to find Solarus the Immaculate in V Rising, we hope that you will be able to quickly eliminate this enemy now that you have everything you need to find him.