I was in a game today, and I saw that somebody playing Vi that had 3 Summoner Spells. I, like the other players on my team, only had 2 Summoner Spells. Where Vi had the third Summoner Spell, we had a square with an 'X' in it.
My question is, how do I get a third Spell? Thanks in advance! :)
I believe you're referring to keystones. When you get to 18, you can put your 18th point into one of the abilities at the bottom of a mastery tree.
What did it do?
That's the image box for your keystone mastery. It's a passive effect you get from your mastery trees when you put 18 points into one of the three types of trees. It appears next to summoner spells in the loading screen and you can only have one. In the case you didn't take a keystone mastery, it is replaced by an X as you described you had seen.
