Interesting. I had been wondering why she could be there in her entirety
Me too.
Ok obviously as a Titan, Ethniu wields a lot of power. An incredible amount of power. Still I belive there is another reason she was able to take on Titania, Odin, and Erlking at the same time. A reason she could compell Ferovax, the most powerful Dragon, and celestial Being.
Ferro stated that his true form would crush houses, and warp reality. So to be in the mortal world he has to leave a lot of his power at the door so to speak. It's the same reason why I think Mother Winter needs her walking stick. It allows her to be on Earth without destroying reality.
I suspect that the Titan armor made by the hundred handers acts much like Mother Winters walking stick. It allowed Ethniu to bring much more, or maybe all over her power onto Earth without destroying reality. It meant she could hit with everything she had while the ones she was fighting could not use all their power.
A very interesting theory! I quite like it. Especially when you consider Titanic Bronze is a alloy of divine metal (so central to reality) and anti-life (something from outside of reality). Perhaps the combination is what allows her to do what most other beings in her weight class cannot. It actually makes a certain amount of sense. Especially when you consider that Angels in order to really "act" have to be bound to physical objects, and wielded by mortals. It is hinted they are similar sorts of beings. Perhaps Ethniu possessed/created a physical body but because it is of the physical world it has physical weaknesses. In fact, I suspect part of the reason Ethniu was happy to have reality break down is because it is easier for her and those like her to be free and not worry about pesky physical laws. So Ethniu then has to create a special weapon, this special suit of armour which allows her to anchor and amplify/focus her power on Earth. But there is of course a cost - it means she is vulnerable to mortal events. I suspect smarter, more defensive beings would never commit their whole self to any one place in spacetime. There is even a hint that this is the whole reason the mantle system in the Queens and Knights exists.
Ethniu never had to give up power to remain in this reality. She has never left it and simply remained low-key until BG. Remember she is called theLast Titan by Vadderung. So she's just the last of the beings of her power level walking around free. There are greater powers locked in Demonreach according to Harry's thoughts while confronting the HWWB on the boat.
I understand what you're getting at here but I don't think that's
quite it. Yes, she is the last of those old beings walking around (supposedly). Always silly to call yourself something absolute, what happens if there is another Titan? Does she then call herself "The Almost Last Titan" or "The Second Last Titan Pending Further Reveals"? Doesn't have the same feeling does it.
But I digress. We don't yet understand why Vadderung had to give up most of his power in order to be active in the mortal world. We only know that this happened because of certain WOJs that specifically tell us about it. It's clear Hades and others like him didn't choose that, but as a consequence lost influence on the world. Which Vadderung clearly believes is worth fighting for, perhaps even worth dying for. But Jim has stated repeatedly the Gods had to back off from the mortal world because they mostly sort-of fulfilled their part of things and either were retired and became pro-wrestlers (because of course Jim), or chose to inhabit the timestream differently, or retire to the Nevernever, or were killed or locked-away in some cases.
I suspect Ethniu hid in order to not be killed or locked up (again - she might have a bit of PTSD over it, interesting choice Harry), but clearly also because she felt she had the divine right to rule but didn't have the power to support it - not just in terms of her divine might but also her army. I suspect she didn't even have access to Titanic Bronze or her father's Eye - almost certainly the work of the Outsiders/HWWBt and/or the Black Council. But she clearly was plotting her revenge for centuries if not millennia. She and Corb were building an army hoping they could take on mortal world but knew there would be other powers that would contest them, namely their old enemies the Fae/Sidhe.
Consider that most of those being who didn't chose to step-back from reality or give up most of their power were probably taken out or locked-up. Question is, who was doing all that? The Wizards seem like an obvious choice...but probably not unaided.