Quest request at Lord Agrippa on a regular basis, until you get the quest 'Harvest Time'. This quest has no time limit, cannot be lost by dying (pk or otherwise). Each phase of this quest is completion of other quests, at 5 other quest masters: Lord Vashir, Lady Undya, Lord Vendredi, Lord Telleri and Lady Templeton. You must have the 'Harvest Time' quest from Lord Agrippa first before doing the others so it counts.
Go to each of the quest masters (any order) and try to get their harvest quests. Once you complete all 5 harvest quests, check quest status of 'Harvest Time' to make sure you aren't missing any, then go back to Lord Agrippa and quest complete to get the mark.
Rewards: