Hey folks!

I have probably just managed to break my game:

While wandering around in the area east of Tenebrae I stumbled upon the changeling pit where the Executioners Hoods grow. Thinking "those will come in handy later on" I took all of them with me. Concerned with encumbrance I stored away all of them together with some other items in a container at Mythrans house. But lo, little did I know that container content does NOT persist in Ultima 8. I embarked on a quest to find the Slayer weapon and ventured to steal the Dagger for the Necromancers. Several hours later all of my earlier saves games were overwritten. Now, Vividos wants some reagents including one sample of Executioners Hood.

With the original samples in the changeling pit all gone (and the don't seem to regrow, right?) are there any other sources of this reagent available at this stage of the game?

If not is there any other way to continue with the game? It is possible to cheat them into my inventory?

Dunno if your problem has been solved, since you posted this weeks ago.
Anyhoo, I find it strange that the items you stored were gone. I don't recall this ever happening to me, although admittedly, I always use an uninhabited house to store things. Strange that npc's would just take your stuff, especially good ol' Mythran.

If those Executioner's Hoods don't regrow then I am afraid you need to start over, since I can say in 100% certainty that you won't find any other Executioner's Hoods at this point. You find loads of them after you have access to Open Ground and you'll need to give Vividos an Executioner's Hood in order to get access to this spell.

The only other thing you can do is cheat. If you were to use the in-game cheat to call up the spell menu and use Open Ground on the catacombs entrance (building in the graveyard, behind Vividos' house) then you'll be able to reach other areas where you can find more Executioner's Hoods.

As far as storage goes, I like to store all my stuff in the first house I see when entering Tenebrae from the docks (where whatshisname is executed). Never have problems there.