Kingdoms of Amalur: Re-Reckoning

This achievement unlocks in normal game play when you kill the 25th Niskaru. Niskaru are present in significant numbers in the main story as well as faction quests and side quests.

Any Niskaru killed counts towards the achievement. There are various types—Niskaru Hunter, Niskaru Bloodhunter, Niskaru Tyrant, & Niskaru Horrinox. A number of bosses during the game are 'named' Niskaru: Azaghaal—The Great General main story quest, Fyragnos—Worth Fighting For side quest in The Red Marches, Enfamanir—Shadow of Enfamanir House of Ballads faction quest, and Balor—Breaking The Siege main story quest.

The Warsworn faction quest has, perhaps, the greatest concentration of Niskaru. However, one would unlock the achievement just by playing through the main story. The 30 second video below shows the achievement popping after the 25th kill during a Warsworn faction quest, The Road Patrol:

You should get this normally by doing the main story and Warsworn guild quests. But if you, for some reason, want to grind/hurry this up there is a side quest in Ayten (The Wolds), given to you by Singing Nell, who is just walking around outside.

She will give you the quest The Erathell's Blessing, which is to play 5 stones in a specific order. Now, if you do this correctly 1 Niskaru will spawn, and this can be repeated until you get it right. Playing them wrong is easy, just play them in a circle order, either directon.