The The Splintered Sea Add-on for Besiege has 20 achievements.
Destroy all the crates in The Frozen Path and collect the strange fragment
Destroy all the crates in Ironweave Passage and collect the cube fragment (DLC)
Destroy the imposter in Solomon's Flock
Find a way to retrieve the strange fragment in Argus' Grounds
Conquer the Splintered Sea (DLC)
Break the strange fragment out of the ice and collect is in Relict Frost
Get the axe throwers in Penitent Tower to hit all the training targets
Find a way to open the shell in The Devouring Pit and collect strange fragment from inside (DLC)
Destroy the cargo ship and collect the strange fragment in Serpent's Crest (DLC)
Destroy all the crates in Ambush and collect the strange fragment
Lure a shark into the cage in Feeding Frenzy (DLC)
Open the chests and collect the strange fragment in Webley's Pass (DLC)
Locate and collect the strange fragment in The Last Stand
Destroy the entire army in Midland Patrol, without destroying any tents
Complete All Secondary Objectives in the campaign
Complete All Secondary Objectives in The Splintered Sea (DLC)
Kill all the knights in The Duke's Plea, except the Duke's diplomat, and collect the strange fragment
Kill all the Duke's knights in Aras' Refuge
Make the monks in Grand Crystal play a secret melody
Destroy the monument and collect the strange fragment in Lyre Peak