Professor Onyx

Legendary Planeswalker -- Liliana (M)
Magecraft -- Whenever you cast or copy an instant or sorcery spell, each opponent loses 2 life and you gain 2 life.
[+1]: You lose 1 life. Look at the top three cards of your library. Put one of them into your hand and the rest into your graveyard.
[-3]: Each opponent sacrifices a creature with the greatest power among creatures that player controls.
[-8]: Each opponent may discard a card. If they don’t, they lose 3 life. Repeat this process six more times.
5

Meme with Tiffany Pollard exclaiming, "Angelina Jolie!?"

I'm realizing I have like no idea how much time passes between each set.
Although I am probably overselling the amount of effort required to become a professor on the school plane

Honestly, all you probably have to do is demonstrate a proficiency.
They can't exactly do much in the way of background checks after all.

Given Gavin Verhey's teaser video, Strixhaven's recruitment process does seem to boil down to "You look capable, wanna teach?", yeah.

I conjecture that the so-called "Professor Onyx" is actually Liliana under an assumed name

What could possibly give you that idea?

I mean, to be fair, how much did Hogwarts ever vet this year's Defense Against the Dark Arts teacher? That's probably one of our big touchstones, people.

Do we know if Liliana ever interacted with Kasmina during War of the Spark?

I'm not sold on her. So for every 6cmc walker ever printed that's seen play in standard has either had a +1 that makes tokens to defend itself (Ugin Vraska Elspeth Liliana Golgarruk), or a minus ability that sweeps the board (Chandra, Chandra, and also Elspeth). There's going to be a lot of times where you play her, neg to kill a guy, and your opponent just cracks in and kills her, and you fall even further behind on board. 6cmc cards already require the game to go long and for you not to flood, you can't really justify putting a 6 drop in your deck that you often aren't happy to cast even when you get to 6 mana.

On top of that, the life drain trigger isn't very useful. Any deck that runs a 6 drop walker doesn't really care about face burn, and if you can stick her for several turns you were gonna win anyways. And the lifegain that's contingent on untapping with a 6 mana walker is a bit late to be relevant against aggro.

I'm assuming Lili's been a prof there for a while and has merely been on sabatical this whole time we've known her.

So, if Severus Snape was a woman, he'd be this card?

Add a shock helix (little lightning helix) on a 1 mana spell like duress? Not saying modern viable, but it does raise some interest...

Though doubt I'm buying any magic soon.