The ancient crystal is a drop obtained from revenants found in the Revenant Caves.
Four of these, along with four marble blocks and level 80 Construction, are required in order to build an obelisk in the Superior Garden of a player-owned house.
For an exhaustive list of all known sources for this item, see here
|Revenant cyclops||82 ||1||1/3,253.33|
|Revenant dark beast||120 ||1||1/2,933.33|
|Revenant demon||98 ||1||1/3,253.33|
|Revenant dragon||135 ||1||1/2,666.67|
|Revenant goblin||15 ||1||1/9,773.33|
|Revenant hellhound||90 ||1||1/3,253.33|
|Revenant hobgoblin||60 ||1||1/4,186.67|
|Revenant imp||7 ||1||1/14,666.67|
|Revenant knight||126 ||1||1/2,666.67|
|Revenant ork||105 ||1||1/2,933.33|
|Revenant pyrefiend||52 ||1||1/4,186.67|