The Then and the Now was a legend used to scare Gallifreyan children.
The truth was that the Then and the Now was created when the War Doctor reversed the origins of a chrono-tracker collected from the Then and the Now in its future. That, after reacting with the Psilent songbox, created the Then and the Now in the paradox. The War Doctor then used the Then and the Now to send Alice Obiefune out of the Last Great Time War and back into her own time. The Then and the Now later gained its sentience. (COMIC: Fast Asleep)
The Then and the Now chased the TARDIS from Veestrax to a Sontaran colony world. It joined the fight between two warring Sontaran factions. The Then and the Now took the full brunt of a Suicide Specialist Sontaran's explosive blast. (COMIC: The Judas Goatee)
The Then and the Now followed the TARDIS through the Doctor's stargate to Shada, where it was assaulted by the prison's robotic guards. The Then and the Now defended itself from the robots, which allowed the Doctor and his companions to slip into the prison undetected. (COMIC: The One)
River Song set up a situation where all the prison's security would be directed at the Then and the Now so that they could make their escape. This held it off for a time, before it once again chased them down on the planet Clundanius XI. (COMIC: Downtime)
When the Doctor and his friends escaped, it next found them when the TARDIS was being assaulted by the Malignant. The Doctor moved his TARDIS sideways as the Then and the Now came for them, causing it to instead collide with the Malignant.
The Then and the Now escaped again, and traced the Doctor to Lujhimene. The Then and the Now itself was followed by The Squire, who attacked it with a quantum distruptor-mace and a harpoon. It fought back and struck the Squire down, but was unable to capture the Doctor before he escaped again. (COMIC: Running to Stay Still)