Here is the full list of all 12 Guards of the Gate achievements.
Find a legendary item
Buyback an item from the vendor
Change talent on a skill shrine hex
Fully complete a collecting event!
Visit statue 10 times
Complete campaign on normal mode
Complete arena on normal mode
Complete tutorial
Complete the wishing well event
Visit campfire with all heroes above 90% life
Get damage by firebug explosion with all the three heroes
Lower enemy life to zero with 1 hit