Wondrous Item, uncommon
As a Magic action, you can open or close this bottle.
Opening the bottle causes thick smoke to billow out, forming a cloud that fills a 60-foot Emanation originating from the bottle. The area within the smoke is Heavily Obscured.
Each minute the bottle remains open, the size of the Emanation increases by 10 feet until it reaches its maximum size of 120 feet.
Closing the bottle causes the cloud to become fixed in place until it disperses after 10 minutes. A strong wind (such as that created by the Gust of Wind spell) disperses the cloud after 1 minute.
Blind fighting / Skulker and this item is really good early game combo.
Can you open/close the bottle repeatedly to get the smoke to spread faster than it spreads on its own?
If you're opening and closing it in one spot I'd rule it as no, given that it said it only fills a 60ft area when opened. However you can be creative and move around opening and closing it making more 60ft areas of smoke, since it said the cloud remains fixed and disperses after 10 minutes and doesn't say that creating a new cloud removes the previous one.
for a rogue (thief subclass) this would work well to set one of the conditions to Hide, but at 60 foot emanation you'll probably blind everyone else in the party. too bad the smoke cloud isn't smaller radius (10-15 feet) to start
It’s an Emanation, not an area. That means it’s 60ft in all directions emanating out from the bottle (120ft + bottle width/height diameter sphere). Emanations also move with their point of origin, so you can just open it and carry it, surrounding yourself in 60ft + 10ft/min on all sides with smoke.
It’s a great way to blind everyone to force flat rolls, (advantage from being unseen + disadvantage from being blind) if the enemy might have advantage and doesn’t have blindsight (or it would give you advantage if you do), but it also completely prevents casters from casting spells that require sight. That’s a massive boon if you have a martial party going up against casters.