There are five symbols. For now, they'll only light up in Spectator Mode if you hover them with your mouse, and as you said, a few seconds before the minions spawn.
Here's a nice thread about them on Reddit: PBE Strange Symbols Added to SR
And another one: The Enigma of the Mysterious Symbols has been solved ! (even though it's definitely not solved yet. xD)
My guess would be an upcoming champion Teaser. Surr@20 found some files having the name "MM_Bard" or something like that, if I remember well. So, might be a new champ based on the music, a bit like Sona (Riot is focusing a lot on Music recently).
Also, there are a few more teasers with the "same" symbol in two different places.
First, on the Tristana's rework picture: PIC
And also in Amumu's video, at the beginning, on the bottom left corner: The Tale of the Sad Mummy
Also, some **YouTubers **videos featured the symbol. Even though it might be fake, such a teaser has been used for the release of Vel'Koz.
See the videos here:
Personally, I think those videos are just made by the YouTubers in order to get views, but who knows?
Now, there are looots of theories already...
Like, for instance, if you draw a line between all the dots, you'll get a pentagram, which refers to Alchemy, which could lead to an Alchemist champ or something similar. But as a Vel'Koz player said, you'll draw a pentagram whenever you do that with 5 points. :)
Some said that the position of the points could refer to **Cassiopeia's **constellation (which is "kinda" true). So, maybe a Cassio skin or something like that, but I really doubt it.
Some people already tried to get the 5 players of the team near the symbols, but nothing apparently happened.
Most people think it's something about Amumu. Indeed, there was "clearly" a teaser with those three dots in the music video. Some think that he'll get a rework, some think that he'll get a lore update... Personally, if it's something with Amumu, my guess would be on a new champ related to Amumu. Like the one who cursed him, for instance. I'd love that, actually. ^^;
Also, let's not forget Ao Shin, the dragon. The runes in the Rift do look like something that might be related to Ao Shin, the long awaited champion teased in a LoL comic book (can't remember the name), in which Udyr had to face his greatest enemy, or something like that, not sure anymore. Heard something about Ao Shin only revealing himself when **Ionia **is in great danger. Is Ionia in great danger? I do not know, but maybe.
EDIT: Forgot the "New 120 AP Item" theory, in which those five points could be of some use for it. But it's unlikely, since the dots aren't mirrored, which would directly offer an advantage for one side. So, I don't think it's gameplay-related.
EDIT2: Some think it might be a jungler thingy. Like a teleporter of some sort, or a vision-related object. Once again, not mirrored, so unlikely.
Weeeell... Lots of theories, lots of interesting theories... But no one knows for sure, excepted the Rioters... :)
I just can't wait to get some clues on this mystery.
Rioters, please help us solve this amazing riddle~! 
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bard (His face) :)
