Boneripper

Boneripper is Unbreakable and immune to the effects of poison.

A small version of the dreaded warpfire thrower has been built in place of one of Boneripper's arms. This is treated as a breath weapon in all respects. When firing, place the flame template with its narrow end in contact with the weapon's muzzle. Models hit by the template suffer a Strength 4 hit. Units suffering any casualties must take a Panic test.

Boneripper must be deployed within 6" of Thanquol at the beginning of the game. After that, Boneripper is free to move independently (in reality he is telepathically controlled by Thanquol) and will count as a separate unit of a single Rat Ogre for the rest of the game (he is worth 75 Victory points).
Boneripper can never join units. If Thanquol is killed or flees off the table, Boneripper switches off and is immediately removed from the table, counting as a casualty. At the beginning of each of his turns in which the Skaven player wants to do anything with Boneripper (move, shoot, fight in close combat) the player must roll a D6 to activate the automaton. On a roll of 2-6 everything is well and Boneripper acts as normal. On the roll of a 1, Boneripper suffers a malfunction — roll another dice and refer to the Malfunction chart above to find our what's gone wrong. 

BONERIPPER RUNS AMOK!
D6  |  Result
1-3) Boneripper does notbing this turn and is bit automatically if engaged in close combat, The opponent can control Boneripper in his next turn, exactly as if it was one of his models. At the beginning of the next Skaven turn, control reverts to the Skaven player.
4-6) Boneripper does nothing this turn and is hit automatically if engaged in close combat.

Before giving Boneripper to Thanquol as a gift, the Warlock Engineers made sure that their creation couldn't be turned against them in the future. Boneripper cannot charge/overrun if the charge/overrun would take him into base contact with a Skaven belonging to Clan Skryre. He can never pursue a unit containing one or more members of Clan Skryre. He can never direct any of his close combat attacks against members of Clan Skryre. Also, the template of his warpfire thrower cannot be placed so that it could hit a member of Clan Skryre

Unit causes Fear

Some creatures are utterly fearless and will never give up a battle, no matter how hopeless the situation might be. This could be because of the troops' extreme bravery, or because the creatures fighting are not truly alive.
These troops never break in close combat, and they are also immune to panic, terror and fear or any other Psychology rules.
If defeated in close combat (even by fear-causing creatures that outnumber them) unbreakable troops continue to fight on regardless of results.
They may never, however. declare that they are fleeing as a charge reaction, as they will literally die fighting under any circumstances.