I have a reasonably mature vault.

Normally, radroaches can be ignored, Deathclaws splatter themselves against my top floor, and Molerats are harmless unless they appear in an unoccupied room and spread like wildfire.

So just what the ???? happened just now?

I rushed production in a super reactor, failed, got a Radroach invasion.

No problem, right?

After all, the room has 6 * L50 dweller, all with all-10 stats, all armed and the weakest weapon among the lot is a Victory Rifle (10-14)

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I cannot feed my dwellers Stimpacks fast enough to keep them alive!

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In less than 30 seconds, the room is depopulated.

Strangely enough, the surrounding rooms managed to contain the outbreak trivially????

All I know is that high tier room = harder enemies and fires. I recently upgraded my training rooms and noticed that molerat invasions that started from those rooms turned annoyingly hard. I built small lvl1 medbay room to do rush, fire etc. room objectives and noticed that it didn't take more than couple of seconds for my single dweller to put out fires or kill radroaches/molerats that came in there. So maybe room size affects too.

I had the same thing happen. I sent wave after wave of dwellers to kill a silly little Rad Roach infestation. Finally had to send unarmed dwellers when all the ones with weapons had been killed by a single Roach.

Yesterday, I had my elite guards wiped out by a single deathclaw attack. Yes, I know, "they're DEATHclaws," but every single time in the past, my guards can finish them off with only losing 20-40% HP. Every attack since then has returned to standard difficulty.

Perhaps we are being experimented on...

Room size is responsible mostly for the event HP (larger room = longer lasting events), Room level is affecting HP and DPS (huge multiplier), average dweller level of your Vault is only affecting DPS.

before i reach my vault's top i had no problem with deathclaws even that my dwellwers weren't lvl 50 and without endurance like 8 or so but after areaching 200 dwellers and supposedly make them harder i start losing dwellers on their attack (and the weapons only become better) so i think it's a sum of vault population, dweller lvl(of all dwellers in the vault) and time spend in the game

Ok, interesting.

I had a goal of "kill 45 creatures". None killed yet.

Deathclaws invade. 3 figures, as per usual.

By the time the second figure dropped, my quest was COMPLETE.

So each deathclaw figure represented at least 23 creatures?

You can't make your dwellers harder by leveling them as the creatures level with you. So by leveling up yourself you level up your environment as well.

It's a common feature in Bethesda games and is used to create a challenging game regardless of your progress. In the end it basically means leveling up is useless.

The only way to improve your dwellers in relation to the creatures are weapons.

Costumes are only for production (some) or for show (most). Only exemption are the Wasteland outfits as they provide a huge endurance boost which translates into a huge HP when leveling. But as you shouldn't level in the first place...