Who is the first test cricketer to have taken a catch in test cricket as substitute and what is unique about this particular catch?

W.L. Murdoch of Australia was the first substitute fielder to take a catch in test cricket.

It was in the first innings of the 2nd test of Ashes series 1884 played at Played at Lord's, London from 21st to 23rd July, 1884. between Australia and England. Murdoch was Captain of Australian Team. When W.G. Grace of England sustained a finger injury, Murdoch, helped England by fielding in his place and took the catch of H.J.H. Scot at the bowling of Steel.

The unique about this was that Murdoch was the captain of rival team.