In the Tainted Darkness event the Islands of Shadowmoor support is carding the opponent’s first card but the rules are that unless specified otherwise that discard is applied to the last card in a player’s hand. Either Islands of Shadowmoor should explicitly say it discards the first card or the effect should be updated to discard the last card.
Discard
Cards that use the Discard ability will, by default, discard the last card in hand (either yours or your opponent’s, depending on the card). If it would Discard any other card, it will be specified in the text (“Discard the first card”).