The reason for this price spike, which comes just as the card is about to rotate out of Standard, is pretty straightforward
Vanquishers Banner Getting an anthem effect isnt what you want from a 5-mana artifact
Aside from the fact that this is probably the most cataclysmic event the game has ever seen story-wise, it's also the first set in an extremely long time to introduce a new card type, as well as just generally being jam packed full of crazy legends, double sided cards, new mechanics, and more
With many formats revolving around 20 life, dealing 10,000 damage is way overkill, and it's most likely there for flavor since Final Fantasy 's Jumbo Cactuar also has an attack called 10,000 Needles that deals that much damage to one party member
Some buyers chase a card because the character means something special
Whats the theme, Sea Creature Voltron