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.
Spider Presence - Reduces the damage and armor of attacking cavalry by 25%. (Passive ability)
Mirkwood Spiders are used as light cavalry, almost as scouting units. They have little armor and very quick deceleration along with high vulnerability to trample revenge damage. However they can be used effectively against swordsmen and to some extent against archers. It is very important to try and put Sauron's Influence on the fortress to be able to recruit them for free. However, it is key to note that you cannot use Endless Horde on them; therefore, it is maybe more interesting to switch to some type of Dol Guldur Orc before using the power.