Varterral

Varterral are huge spider-like creatures that stand on five massive legs that allow them to move at surprising speeds.

Background
These deadly guardians were created by elves in the time of ancient Arlathan, and are now often associated with that lost elven culture and the times that have been lost to history. It is said that the varterrals were created from the elements of rock, fallen trees, wind, and rain.

The first of these creatures were created to guard an ancient city beloved by the elven god Dirthamen. It is said that if charged to guard something, a varterral will stay alive no matter how many times it is struck. They attack with speed, fast strikes and venomous spit.

Witch Hunt
A varterral appears as the final boss of Witch Hunt, the final DLC for Dragon Age: Origins. The creature's presence in the Dragonbone Wastes comes as a complete surprise to everyone present and is never explained, though it presumably has something to do with the Eluvian hidden deep beneath the Wastes.

Dragon Age II
A varterral is fought in the Varterral Hunting Ground in Sundermount during the "Mirror Image" quest, and again at the same cave and place during the "A Murder of Crows" quest; Hawke sees the varterral for the second time and says "Didn't I already kill it?" If Merrill is present in the party she will explain that as long as the varterral has a duty to guard something, it will stay alive no matter how many times it is defeated.

Trivia

 * This is the first creature for Dragon Age II to be described on BioWare's site, and one appears in the Destiny trailer.
 * Although they are never referred as such in game, the BioWare website and concept art refer to the beasts as "Striders".
 * The Witch Hunt Varterral has the distinction of being the final enemy of Dragon Age: Origins and its DLCs.

Bugs
When you are trying to kill the Varterral in Dragon Age II, it may get stuck under the big boulder near the exit passageway stopping you from getting any loot.