How is it that people who enter the isles are dead the moment the mist touches them, but Lucian is able to have a full on battle with Thresh without consequences?
How does one break this rule? Who is and isn't able to enter the isles?
In the card game legends of Runeterra, Demacians were able to enter the shadow isles without dying to the mist. They only died from its inhabitants there though . Maybe when the Ruined king unleashed the curse there was some sort of damaging magic but 
still lives while his friends are corpses.
Maybe the mist is some sort of toxic gas that slowly became less toxic over the decades? and Yorick's vial is some sort of a gas mask? but idk
The same reason Senna can forge a massive gun in Thresh's lantern, make a new outfit and style her hair and bring it all with her when she escapes. Same reason a small tribal community of Romanesque warriors who live on Mount Targon can also happen to look very Asian, and can develop high tech guns and still somehow be wiped out by spear chuckers... and also develop said guns in the spirit world.
Riot's Lore team is all over the place and inconsistent as hell.
The book has a diagram showing how the Isles makes intruders slowly go paranoid and then insane as insidious spirits gnaw at their soul
Stage 1 : Anxiety Stage 2 : Paranoia and hallucinations Stage 3 : Deceased people the victim knows begin to take form Stage 4 : Leeching spirits at the victims spirit until they let go Stage 5 : The victim weakens until their physical form wastes away or becomes a vessel for attacking spirits
Pretty sure he shoots the mist
I would love some more insight on that! Maybe Interlocutioner can chime in? Hopefully!
From what I know the mist has to claim you. Odds are Lucian avoided the mist. We know you can go the Shadow Isles but you won't last long because you will either die by the hands of an undead, or the mist will find you. I read the Realms of Runeterra a book and it talks about the mist (of course) and it does seem like that's how it works. But during the Harrowing the mist grows more power.
Also after reading the book I'm super hyped for the future of Bilgewater and the Shadow Isles after what I read. Along with the Ruined King returning and a game based around it I'm super hyped. (little bit of a teaser as to what might happen is Kindred might get involved with the Shadow Isles)
How is it that people who enter the isles are dead the moment the mist touches them, but Lucian is able to have a full on battle with Thresh without consequences?
How does one break this rule? Who is and isn't able to enter the isles?
So, while the Mist surely is deadly, it doesnt cover every single bit of air and land in the SI and people can walk on the areas that arent covered by it. This "rule" that people die the moment they reach the Isles is actually just a common assumption that isnt truly supported by lore. While Lucian can just kill undead who attack him, we have A LOT of other examples of people being able to walk on the Isles before predatory spirits attack them:
“Help… me,” begged the shipwrecked man.
Yorick couldn’t say how long the survivor had been lying there, bones broken, bleeding into what remained of his wrecked sailing vessel. He had been moaning loudly, but his cries were drowned out by the multitude of wailing souls that haunted the isle. A maelstrom of spirits gathered around him, drawn to his flickering life force like a beacon, hungry to reap a fresh soul. The man’s eyes widened in horror.
He was right to be scared. Yorick had seen what happened to lost spirits taken by the Black Mist, and this—this was warm flesh, a rarity in the Shadow Isles. It had been how long—a hundred years?—since Yorick had seen a living being? He could feel the Mist on his back quivering, eager to wrap this stranger in its cold embrace. But the sight of the man stirred something in Yorick he had long forgotten, and whatever it was would not allow him to surrender this life. The burly monk heaved the damaged man onto his shoulders and carried him back up the hill to his old monastery.
Last Rites, Yorick color story (How long Yorick has seen a living being doesnt reflect how long anyone has been on the Isles cause it's an entire archipelago and he cant look over the entire thing at once)
Elise found a desperate, debt-ridden captain willing to bear her and her fellow pilgrims across the ocean. The ship sailed for weeks until a craggy island loomed from seething banks of black mist. Elise came ashore on a beach of ashen sand and led her followers deep into the island’s haunted depths like lambs to the slaughter. Many were stolen away by spiteful wraiths, but half a dozen remained by the time they reached the web-wreathed lair of the Spider God.
Elise bio While many of them died the fact that they reached Vilemaw's lair still proves my point. The same thing happens in her color story(even though there you could argue Elise was using her powers to protect them, but on that bit of the bio she was a normal human).
Icy waves crashed on the bleak shore, red with the blood of the men Hecarim had already butchered. The mortals he had yet to kill were retreating over the beach in terror. Black rain doused them and stormclouds boiled in from the mourning heart of the island. He heard them shouting to one another. The words were a guttural battle-cant he did not recognize, but the meaning was clear; they actually thought they might live to reach their ship. True, they had some skill. They moved as one, wooden shields interlocked. But they were mortal and Hecarim savored the meat-stink of their fear.
No One Lives, Hecarim color story
Now, to mention Realms of Runeterra:
There are people who make a living out of entering on the Isles, looting magical items and selling them. That would be impossible if the Mist instantly killed everyone who got there. It's extremly risky but not instant death. There's also a part that explains what happens over time with you if the spirits dont kill you, and by the description you can see that it's not an instant process as people seem to think
As you can see, Lucian didnt break any rule, because there isnt any rule saying the Mist instantly kills people who enter in the Isles, as opossed, we have many examples of people who were able to enter there and when powerful spirits didnt hunt them, some even got out of there alive. This is just an assumption that people tend to make and it's wrong.
The mist must claim you. I believe this means that the mist literally takes you over. Or, the mist's creatures kill you and drag you back to it.
The mist grows weaker and then goes out during the Harrowing to collect more souls and grow stronger.
I believe that there is certain levels of magic that protect against the mist itself. I would also go as far to say as it is something like an enchantment that wards of the power of the mist. From the lore I've read, there was always something that stopped whoever from being consumed amulet 
Pistols. Though its not confirmed im willing to bet that there are ways to combat the mist that are readily available and trinkets like lucians pistols are some of them.
There are no such things as "rules" in this universe. Everything can happen if the writers want it.
The mist just slowly saps your life force. If you stay on too ling you might die, not immediately.
And this is not the only instance where the mist has no rules. Remember back when the mist reached bilgewater? Yeah, how the hell were they able to drive it back? How...what kind of damned magic was able to keep the mist back? Even tho the story is kinda old (5 years ago) so it might not be canon but still, since bilgewater has no newer updates on this matter, then it should count as canon.
They don't die instantly. In Yorick's short story, he finds a wounded fisherman stranded on the isle. The man is alive but terrified, and Yorick takes him to an abandoned house or something; he ends up dying from his wounds and becomes a ghoul.
I get the impression that life and living beings attract the malevolent spirits of the Mist; in Maokai's short story he defends a blooming flower from being destroyed by spectres. The Mist doesn't actively kill a living person. Possibly leeches life and vitality from them, but that's it unless someone is harnessing its power directly.
