Fugaku Uchiha was the father of Itachi Uchiha and the leader of the Uchiha clan when he was alive. Fugaku Uchiha was killed by Itachi Uchiha during his massacre of the Uchiha clan ordered by Danzo.
We have seen that Fugaku Uchiha possessed the Mangekyou Sharingan. When he told Itachi about the stone tablets, he revealed that during the great ninja war, he awakened the Mangekyou Sharingan. But what were it’s abilities? How strong could his Mangekyou Sharingan have been? That is something that has been left untold.
Fugaku Uchiha’s Mangekyou Sharingan
When Fugaku was talking to Itachi near the stone tablets, he made it very clear why he did not reveal to everyone that he has the Mangekyou Sharingan. He said that had he revealed his Mangekyou Sharingan to everyone, people of the Uchiha clan would have asked him to control the nine tails with his abilities and wage war against the rest of the hidden leaf village. And so, to stop that from happening, he kept his Mangekyou Sharingan a secret.
Fugaku Uchiha’s Mangekyou Sharingan abilities
What has been left untold are the abilities of his Mangekyou Sharingan. Fugaku was a powerful Shinobi no doubt. That is the reason why he was appointed as the leader of the Uchiha clan. Everyone had only seen the abilities of his normal Sharingan and that was enough for him to be appointed as the leader.
Uchiha’s are an intelligent clan. Having a Sharingan doesn’t help unless you are clever enough to use it in an efficient and productive manner. Being the leader of the clan, Fugaku must have been outstandingly smart.
Fugaku said to Itachi that had he revealed his Mangekyou to everyone, they would have asked him to control Kyuubi with it. That means that he believed that he was strong enough to control Kyuubi with his visual prowess. Controlling Kyuubi is not possible with just a Mangekyou Sharingan, you have to be strong enough to do it too.
This means that Fugaku believed that he could control Kyuubi with his visual prowess. He was a clever guy, so this must have been true. Other people whom we have seen controlling the tailed beast are Madara, Sasuke, and Obito. Both Madara and Sasuke had their complete Susanoo’s by the time they controlled Kyuubi. Obito was an exception, he did not have the Susano because he did not have both his eyes. Yet he was very strong.
What I am trying to get at is that Fugaku Uchiha also probably had the complete Susano. It does make sense given how confidently he accepted that he could control Kyuubi if he wanted to which means his visual prowess and abilities as a Shinobi were outstanding.
Fugaku’s Mangekyou had Amaterasu?
I might be extrapolating things a bit here, but we have seen that Itachi had Amaterasu. Sasuke also awakened Amaterasu with his Mangekyou Sharingan. So, it is possible that Fukagu also could summon the black flames – Amaterasu. This is a bit of extrapolation but since his abilities as a shinobi have not been revealed, extrapolation is all that can be done to arrive at some conclusion.
So, we can arrive at a few abilities of his like the Susanoo and the Amaterasu, but other possible abilities are still hidden. One thing is for sure that Fugaku Uchiha really was a very strong guy (though he may not be stronger than Itachi or Sasuke since they were prodigies).