Oblivion Walker is an achievement/trophy in The Elder Scrolls V: Skyrim. To obtain it, 15 Daedric artifacts must be collected. Note that due to updates, one need only to have possessed 15 Artifacts (not including the Skeleton Key). They do not all need to be in the inventory, or even in possession, one just needs to have finished the requisite quests and obtained the items at some point. For example, one can read the Oghma Infinium without penalty. By the same logic choosing to kill Sinding in "Ill Met by Moonlight" appears to grant two artifacts for the purpose of the achievement. This is because the Ring of Hircine is given to the dragonborn during the quest, but replaced with the Savior's Hide at the end.