Armor Cores, Weapon Cores, Etheric Light, and Treasure Keys have been converted to consumable items which are used to grant either House of Judgement or Queen's Wrath experience, depending on the item. For more information on how these activities and items function in year two, you can check out the following help article: https://www.bungie.net/en/Help/Article/13700 The Challenge of Elders is the new weekly Prison of Elders activity. You can purchase a Sigil from Variks and earn points for weapons and gear. This help article can shine some light on that: https://www.bungie.net/en/Help/Article/44552