Time case

The briefcase is an object of great mystery in the IDW portion of the Generation 1 continuity family

There's nothing particularly conspicuous about the small, sturdy, golden briefcase that Brainstorm owns, beyond the fact that whenever anyone else sees it on his person outside of his lab, it's always safely handcuffed to his wrist. He is known to use it to transport around the various "Unmentionables" he has created in his laboratory, and takes delight in teasing anyone who asks about its contents with a mixture of bizarre facts that make it impossible to tell if anything he's ever said about it is true.

Fiction

IDW Generation 1 continuity

Brainstorm took his briefcase to Room 113 to plead another case for his Unmentionables before the Ethics Committee. When he passed Ironfist and Skyfall on the way, they asked about the case, and Brainstorm gleefully laid out the details of the "Malevolent Counterintuitive Pathogen" it contained which would be released if the case was ever opened. Skyfall dragged Ironfist off to the Exit Rooms before he could find out whether Brainstorm was joking. Bullets

Brainstorm brought his briefcase with him when he joined the Lost Light, justifying its exemption from security checks by Red Alert with the "almost certainly serious" claim that if it was opened outside of a controlled environment, it'd be "end of the world". Liars, A to D Part 1: How to Say Goodbye and Mean It When a Sparkeater got loose aboard the ship soon after its departure from Cybertron, Brainstorm used his briefcase as a shield against the beast, and it coincidentally decided to leave him be. Liars, A to D Part 3: The Chaos of Warm Things

Later, Chromedome claimed he'd heard that Brainstorm opened the briefcase on Hydrus 4 and "the sun went out", to which Brainstorm replied that was a filthy stinking lie: It was actually Hydrus 5. Rules of Disengagement Stories like that make the briefcase something of a legend on the ship; when a disbelieving Swerve saw that Brainstorm had brought the case to a funeral for the ship's fallen crew, he decided that someday, somehow he would open it to see what would happen. The Gloaming

On Luna One, when a bemused Rodimus asked if Brainstorm was going to put a "Point One Percenter" spark in the briefcase, Brainstorm remarked that he hadn't considered it, but that the prospect blew his mind. The Fecund Moon A short time later, during a battle with Tyrest and Pharma, a stray laser blast from Pharma severed the handcuff holding the case to Brainstorm's wrist. Arm the Lonely Brainstorm was subsequently among those sent into convulsions by Tyrest's killswitch; after recovering, he reclaimed his case, but became suspicious that someone had opened it while he had been unconscious. He requested that Swerve keep an ear out for any chatter regarding this, with the grim warning that opening the case when he was not around was not a wise thing to do. The Sound of Breaking Glass