The Thijarians were an ancient species from the planet Thijar. They evolved themselves to become the perfect assassins. They communicated by projecting their speech telepathically, which could be painful to experience for anyone, including races like Time Lords. However, this was only a means of disorientation; they could speak telepathically without the painful experience if they choose to. They had metallic heads, with fourteen eyes (seven on each side), and tusks. They could teleport instantly to any location, using miniaturised transmat locks to keep people away.
After their planet was destroyed, and the last two Thijarians were unable to grieve or honour those they had lost, they left behind their old ways. The Thijarians' new objective was to bear witness, across time and space, to those who had perished. They sought to honour those who would otherwise have died alone.
They carried a sample of the DNA of the deceased inside their hive as a memorial to those who they bore witness to.
The Thirteenth Doctor, Graham, Ryan and Yaz encountered them whilst travelling to India in 1947, where the Thijarians were waiting to witness the many casualties from the Partition of India. The Doctor was not aware that their planet had been destroyed, or that they were no longer assassins, until later. She had only heard of the Thijarians and their former reputation before this encounter. (TV: Demons of the Punjab)