coral obsidian
What is it?
Achievements that are available for 2 days, then they're gone. If they do well, they could be reused or added into the base game. If the achievement turns out to be bad, then the impact is minimal as it only lasts for 2 days.
Potential Usages:
This would be used to include achievements that would be suited for more niche playstyles that wouldn't fit well into the base game, but are still fun to experiment with. They could also used be for more complex achievements that are too in depth to be a regular achievement.
These achievements could also be used to test potential achievements, if you wanted to add something, but are not sure how it would actually play out, make it into a limited achievement and observe how the players play with them. This is different than events, as events require a whole new mode.
Example:
Limited achievement: Highest altitude with 
while only place pieces during the invisible period.
- Cool concept that might hold potential, but seems a bit niche.
-> If it does well during the limited period, then it could potentially be added to the base game.
-> If not, then at least it was tried.
These achievements would be unranked and archived after their duration, but they could be potentially revived if enough people enjoy it.
echo tide
it's not that much work to make an achievement
was just 
lol