The Cruciform was a massive intergalactic construct created by the Guardians not long after the dawn of time, which was used to seed humanoid life in the Milky-way Galaxy and beyond. Hidden from normal reality in subspace, the device had four massive 'arms' that stretched at ninety-degree angles into all four quadrants.
In 2387, the dimensional entranceway to the Cruciform was destroyed by Captain Puto and Captain Lewis of Unity Starbase, to prevent the Borg from using its immense power. At around the same time, a future version of the two Captains travelled back in time and entered the structure just before their past selves sealed the entrance, again to stop the Borg.