In the ancient of days, there was a race of humans who lived on the continent of Malhorac. As the gnolls of the south slowly pushed their way north, laying claim to the lands that would become Kyask, the fear of death began to overwhelm them. Eventually, they prayed to the power of death itself, and death answered in the form of the Summerling god Oldorin.
In exchange for their service as his agents, Oldorin transported these humans from Tol to the Shadowfell. There, they became effectively immortal under Oldorin’s power. The trade-off, however, is that they have lost emotion and sensation. They must drive themselves to increasingly extreme lifestyles in order to feel anything.
The shadar-kai have particularly mastered the spiked chain, not only as a weapon, but as a tool to push the limits and extremes of sensation in an effort to feel anything. They are revered and feared for their skills with the spiked chain and katar.