Obelisk (Clan Citadels)

The Summoning plot is a feature in the Clan Citadels which is unlocked when the citadel reaches tier 5. It is used to gather Minions. You need nothing to train on it, possibly making it the cheapest way to get 99 Summoning. However, this is hampered by the fact that only 10% (1/10) of the weekly resource cap may be expended upon it.

By finding the glowing pillar and selecting it you will generate a field that lifts the tumbled stones in the center of the obelisk to form a portal. After a while, a minion will step from the portal, look around, and then vanish to do your bidding. (They count against the other resources in your citadel, so each minion created helps towards your citadel's maintenance.)

http://images2.wikia.nocookie.net/runescape/images/3/3b/Citadel_summoning_minion.gif

Experience
Players gain different amounts of experience depending on a number of factors such as the level of the plot.