Willie Frazer has died at age 58.
The victims' campaigner had been diagnosed with cancer several years ago.
Tributes have been paid to the Co Armagh man who came to prominence as a campaigner for the victims of republican violence.
Five members of his family, including his father, were killed during the Troubles.
Mr Frazer's father, Bertie, was a part-time Ulster Defence Regiment soldier who was shot dead by the IRA on a farm at Ballymoyer near Whitecross in Co Armagh in 1975.
DUP leader Arlene Foster said Willie Frazer "suffered great loss and campaigned for justice with incredible passion".
"He feared no man. The last few weeks have been tough. My sympathies to his family and all who mourn his passing."
UUP assembly member Doug Beattie said Willie Frazer was a victim himself who had been "shaped by the atrocities perpetrated on his family".
Alliance leader and MEP Naomi Long extended her sympathies to the Frazer family.
TUV leader Jim Allister said Mr Frazer's "passion for innocent victims and desire to honour the memory of those in his own family who paid the supreme sacrifice at the hands of terrorists is beyond question".
“Now his struggle is over it behoves politicians to honour his memory by ensuring that the past is not rewritten. We must all ensure that innocent victims are not forgotten."
Earlier this week the Southern Health and Social Care Trust said it was investigating how a picture of the terminally ill man which appeared to have been taken in hospital without his permission had been posted online.