Any friendly monster taking a Monster Reaction test within 12" of Rakarth will always pass the required Leadership test. If the Beastlord is slain whilst riding his Black Dragon Bracchus, the Dragon does not need to take a Monster Reaction test and will fight on as normal.
In addition, a monster that is beaten in hand to hand combat by Rakarth is assumed to be dominated by the Beastlord and will come under his control. This only applies to ridden monsters, and only once the rider is killed. The monster does not flee, but rather is moved back from Rakarth an inch, and is considered to be a part of the Dark Elf player's army immediately. The monster will remain under Rakarth's control for the remainder of the game. If Rakarth is killed then any monsters under his control revert to the control of their original owner. If Rakarth survives the battle, then any monsters under his control are assumed to be destroyed in regards for working out Victory Points. Note that an enemy does not get any Victory Points for killing a monster enslaved in such a way by Rakarth.