Mirkwood Spiders are light 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. The spiders also apply poison with every attack.
Spider Presence - Reduces the damage and armor of attacking cavalry by 25%. (Passive ability)
The Mirkwood spiders have access to the following upgrades:
Banner Carrier - This upgrade can only be given to a level 1 battalion and will promote it to level 2. At level 2 the battalion can regenerate units when out of combat. Cost: 200
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.