I'm going to be an asshole and skip the Yaga for now(They'll be after the Ivan The Terrible and the newly created Anastasia feats because I actually did her a little dirty considering how they explained the Lostbelt version and how her abilities work).
And instead, I'll go straight to the LB King himself:
FGO Biography:
This is the tsar who appears in Eternally Frozen Empire Anastasia, a 'what if' version of Russia from a timeline that was supposed to have been pruned as it was a dead end for mankind.
That is Ivan the Terrible.
This is Ivan reborn as a Servant, but in the history of his Lostbelt, he existed as the first human to turn into a Yaga.
The people who lived in his empire were all Yaga, and this is because it had become a world too cruel to survive in for those with human forms.
However, in the process they ended up losing something very important they had as humans.
The Ivan from panhuman history may have been a tyrant, but he was a stranger to magecraft.
But when the world of the lostbelt was in danger of freezing over, he summoned a group of magi.
He sought a way for humanity to survive, which came in the form of fusing humans and magical beasts.
However, after they had brought upon the world a new type of being different from humans, the minds of these also began to change and they became a dead end for human history...
They became recognized as a world that should be pruned.
Without ceasing his wailing, Ivan continues his pilgrimage with grief.
----------------------
Upon being summoned to Chaldea, he appears not as the panhuman Ivan, but rather the one from the lostbelt. Usually this summoning should be impossible, but perhaps in response to such an unprecedented event like the scorching, or the editing of the human order, the human order has sought stronger heroic spirits.
Maybe it's because he spent most of his time in the lostbelt sleeping, but his memories of it are fuzzy, and he doesn't really understand why he's been summoned in the form of an elephant.
However he has not forgotten his duty as a lostbelt Servant and as the loser, which is to protect Chaldea.
Ivan's power is first stated by Ophelia to be onpar with a
Divine Spirit's:
We later on get from Caenis that his power is on the level of
Zeus's Human Avatar Form(while we don’t get this from the story itself as LB5.1 and 5.2 was 2 years later, it’s still very much apparent Ivan’s power is exactly similar to Zeus’s in terms of scale and ability). In Gilgamesh's new My Room lines for Ivan The Terrible, he explicitly states that Ivan's Mammoth is equal to
Gugulanna in terms of size and power(Which basically makes Ivan The Terrible casual Country Buster with extreme ease).
Atalante does somewhat back up Ivan's power in different fashion with comparing his
main attack(Creating sheer tempest level storms within his Trunk which you can also say is his main NP Attack we see in gameplay) as on the level of Divine Spirits.
We also see that
here, he’s capable of spamming the attack
consecutively enough that Mash’s Ortinax and her NP shorts out after it which leads to the later issue in the Lostbelt.
Which leads to Ivan being able to keep so many Oprinchnki active for so long(24/7 consistently) should be so impossible, the Laws of Equivalent Exchange wouldn't allow for it.
Ivan's
state is due to him fusing with an unknown Primordial Beast that was buried under the Russian Tundra and further
engorged himself in hundreds of not thousands of Demonic Beasts for 400+ Years
Ivan The Terrible actually is able to
understand the Foreign Priestess(most likely due to Menta Schisml) but is able to stop the Lostbelt Tree from taking root(Due to the
Emergency Prerogative which is capable of controlling the very land of Russia) , which is why Kadoc executes his plan to get him removed from his power as a Lostbelt King.
Almighty Supreme Authority: A(It's translated as Emergency Prerogative and it's funny it doesn't mention how ridiculous it is in story since it was able to
stop the ToE from taking root in general till Anastasia took the Crown and the Lostbelt King spot and it was all but implied that if they tried to
root the ToE any earlier, Ivan The Terrible can use it to destroy it.)
Supreme submission and obedience through fear that Ivan sought in order to control the nobles.
The intimidating aura he puts against everyone in his surroundings creates a heavy pressure that reduces all their stats by one rank.
A regular person would find it difficult to keep their eyes open under this.
This skill can be negated by Charisma with rank B or higher.
Now for the basic abilities of himself and his steed? Himself? Ivan is honestly weird with this
Zveri ・ Krestnyy Khod
The Beast that Accompanied Me On My Journeys
Rank: A+
Type: Anti-Unit (Oneself)
Range: 1
Maximum Targets: 1
The cross procession of a divine beast. The march towards the kingdom of heaven that the tsar will eventually reach (or so he believes).
This means standing in the tsar's way is treason and blasphemy against God.
Ivan temporarily returns to his old form as a giant beast and mercilessly destroys his opponents.
Ivan is capable of destroying towns and villages to the point it's like it's been hit by a
natural disaster and comes off as if the
Tunguska Event has
happened
The Event itself?:
The Tunguska event was a massive ~12 megaton[2] explosion that occurred near the Podkamennaya Tunguska River in Yeniseysk Governorate (now Krasnoyarsk Krai), Russia, on the morning of June 30, 1908.[1][3] The explosion over the sparsely populated Eastern Siberian Taiga flattened an estimated 80 million trees over an area of 2,150 km2 (830 sq mi) of forest, and eyewitness reports suggest that at least three people may have died in the event.[4][5][6][7][2] The explosion is generally attributed to a meteor air burst: the atmospheric explosion of a stony meteoroid about 50–60 metres (160–200 feet) in size.[2][8]: p. 178 The meteoroid approached from the east-southeast, and likely with a relatively high speed of about 27 km/s.[2] It is classified as an impact event, even though no impact crater has been found; the object is thought to have disintegrated at an altitude of 5 to 10 kilometres (3 to 6 miles) rather than to have hit the surface of the Earth.[9]
The Yaga later on states that he regularly does this whenever he
wakes up
The resulting
Thunderclaps from the Lightning attacks aimed at Golem Keter Malkuth is enough to level the buildings wholesale
Defensively, The Mammoth is capable of taking attacks from Atalante Alter, Beowulf and LB Anastasia with no damage at all. Even a fully grown Golem Keter Malkuth can’t even
damage Ivan’s hide despite the sheer trouble it caused at it’s weakest level in Apocrypha(caused a major headache for multiple A and B Rank Servants).
Fate/Apocrypha material - Encyclopedia: Royal Crown, the Light of Wisdom [Noble Phantasm], p.166 [T]
Royal Crown, the Light of Wisdom [Noble Phantasm]
Golem Keter Malkuth. The Noble Phantasm of Caster of Black, Avicebron. It is not something that he created while he was alive; rather, it is his unfulfilled dream that was turned into a Noble Phantasm. It is an imitation of the “first human Adam”, a Reality Marble which has been given life of its own.
Just by stepping on the ground, it is endlessly supplied with prana, and at the same time it encroaches on the surrounding land and turns it into “Eden”.
Its full height was fifteen meters at the time of its birth, but this is only the measurement of “when it was born”. As the span of Eden expands, the giant’s size will change and multiple, until it finally exceeds a thousand meters. It is said that “a fifteen-meter-sized giant could be easily defeated by even a Servant with average C rank stats, but as expected, one at the size of a thousand meters would be impossible to kill without a group of first-rate Servants. And, incidentally, the stomachs of the people tasked with covering up it all up would die.
The reason Jeanne and the others were so frantic against the giant when it was still fifteen meters tall is because they could tell that the speed of its growth was extraordinary, and if their response had been even a bit slower, the giant would have grown to thirty meters and displayed even harsher resistance.
Ivan's... steed... Mammoth thing is the literal size of a
Mountain
Kadoc notes he can cause an avalanche with a single step and can slam his Trunk down with the force of an Anti-Army Noble Phantasm.
It takes Salieri’s
Noble Phantasm working at full blast to even begin reducing Ivan’s natural defenses. And it takes him dropping his
Lightning Guard going for slamming his trunk on Mash for Musashi to cut through it with her
Noble Phantasm.
Chyornyj Oprichniki
The Black Hounds Crawling Out of My Dreams
Rank: B
Type: Anti-Army
Range: 1
Maximum Targets: 500
Chyornyj Oprichniki
The royal guards Ivan used along with his Almighty Supreme Authority.
These troops had the authority to steal land and property from the nobles, and they could be considered Ivan's arms and legs.
They wore black, rode black horses with black gear, and hung dogs' heads on the necks of their horses.
At the same time they carried brooms made with animal hair which had the meaning of 'attacking traitors like dogs, and cleaning them out with a broom'.
This is a passive NP, but its activation is peculiar, as it requires Ivan to be unconscious.
Once he falls asleep, his troops with torturing tools in hand will start appearing everywhere to eliminate the enemies of the tsar.
They are similar to vengeful ghosts, and as the tsar's coercive power grows, their morale and fighting ability will also grow.
They may have great strength in numbers, but they are weak individually.
As stated in the Materials,
Oprichniki as a whole requires Ivan to be asleep for them to exist and it was primarily the plans of Kadoc and Rasputin to keep him asleep as they can use the Oprichniki for their plans better
There's also the fact that their strength goes higher the closer they are to Ivan due to gaining power in their faith to him to the point of being on par with a Servant
Their horses are equally strong enough to trample Yaga underfoot(First mention
here, and actual scene of it happening to an Elder Yaga
here)
Oprichniki in the Lostbelt is able to take 6-9 Magic Rifle shots and still be able to fight(in comparison outside of the Lostbelt where Goredolf can kill a few with his Magic Gun with no issues.) and requires Paxti to score a headshot to kill the same one.
And
finally, The Oprichniki whenever they are recalled by Ivan waking up are able to pass on their information to him even while he's asleep(as Anastasia let loose her betraying him to one of them)