The sack of effigies is a cape that can be obtained as a reward from Treasure Trails.
The drop chance of the sack of effigies is approximately 1/46,080 per reward spot from hard clues, and approximately 1/29,542 per reward spot from elite clues. As both hard and elite clues have an average of 5 rewards, the per-clue chance of receiving at least one sack of effigies is approximately 1/9216 (0.01085%) from hard clues and approximately 1/5909 (0.01692%) from elite clues. For a more detailed breakdown of drop mechanics and calculations, see Treasure Trails/Rewards.
There is a server-wide announcement when someone acquires this from a Treasure trail. The announcement refers to it as an "effigies cape".
The examine text "Aww! My very own effigies cape" is a reference to how ancient effigies were used to train tedious or slow skills because they were easily obtained and gave a lot of experience.
This item is a reference to the time before ancient effigies had their drop chances drastically reduced and limited players to owning no more than 5 at a time. Prior to their update on 14 February 2012, players would stockpile ancient effigies to quickly train expensive and/or time consuming skills.