The problem is that you can't cast an open spell in Castle British. It is too high a level and the blackrock gem prevents you from casting this spell. You need to cast an open spell in another dimension, leave the cursor active, return to Castle British and then click on the chest. May take several tries, because the spell doesn't always work, so save before casting the spell.

there's also a Mace (i think) of Open somewhere that i believe activates when you hit a door or chest
if i recall its fairly late in the game tho

Edit: nvm the above wont work since it wont work on chests


i also dont remember if the chest is one that can be destroyed,
if it isn't it will be called massive chest instead of chest

The Cudgel of Entry doesn't work on chests, unfortunately.

The only two ways to open the chest that I know of are:
* As mentioned by Stryx, casting the Open spell in another dimension (through the gem), leaving the spell target as the cursor then walking back through the blackrock gem all the way to the chest in Lord British's room, then clicking on the chest
* There is a Scroll of Open in one of the alternate dimensions, which is very late-game, but should also work to unlock the chest

yeah now that you mention it i remember tryin the cudgel and being disappointed that it didnt work
my bad, guess thats what happens when i havent played in about a year and a half

This actually is a well known Easter Egg in the game. Lord British is keeping a naughty book in the chest and there could also be some potions as well.

The book is in Nystul's chest. LB just has a few gems and eyeballs.