Mirkwood Spiders are light cavalry units that are recruited from Dol Guldur to serve Mordor. They have the passive ability Spider Presence which reduces the damage and armor of attacking cavalry by 25%. If Dol Guldur is under Sauron's Influence then the spiders can be recruited for free, if not they cost 300. It is key to note that they are not affected by the spellbook power Endless Horde.