Combining two runes will give you a random rune of the same tier. Combining 5 runes will give you a random rune of the next highest tier. You cannot combine 5 tier 3 runes at once (as there is no tier 4).
The rune combiner is random. It doesn't matter whether you combine marks, glyphs, seals, or quintessences -- they all work with each other, and the output can be anything.
The idea is that, generally, runes themselves are priced such that it is slightly cheaper to generate a tier 3 runeset from your old runes than by buying the tier 3 runes on their own. For instance, the level 1 +AD rune is 15 ip, while the level 2 +AD rune is 80 ip -- if you bought 5 of the level 1 runes, combined them, and wound up with a level 2 +AD rune, you would have saved yourself IP.
Unfortunately, as mentioned earlier, this process is completely random. If you have tier 1 and 2 runes, by all means, combine them to higher levels; the rune combiner is just a way to possibly recoup the IP you spent on runes you no longer find useful, but it cannot be counted on to give you what you want. You might luck out and combine 2 random runes into a +Dodge quintessence (very expensive!) -- or you might get a mana/5 Mark (with a tertiary stat, and probably not worth the IP cost of your combined runes).
It's very much a gamble, and you don't get anything exclusive from it, so use it if you need to get rid of T1 / T2 runes, and safely ignore it otherwise.
Season 4 Changes
Firstly, tier demotions do not happen immediately at 0 LP. First, you get a warning, then if you continue to lose games, you will then drop the tier.
Also, Shielding and MMR-based demotions have been added.
Shielding means that you are immune to dropping out of the tier for your first N games once you enter the tier. (TODO: Find the exact number)
Also, the tiers are more closely tied to MMR (your [M]atch[m]aking [R]ank). If you play well below the MMR of others in your tier, you will get a demotion warning in your profile, and then drop the tier if you continue to play badly.
For a hypothetical example of this, imagine that the MMR of Silver V is 1500, and the MMR of Gold V is 2000. If you are in Gold V and playing with a MMR of 1570, you will probably get the warning.
Season 3 System
First, the official sources: (Announcement / Image Only / FAQ / FAQ Mirror)
There are five normal tiers and then the challenger tier. Each tier has five divisions. When you win a game you get league points and when you lose a game you lose league points.
When you have 100 league points you will enter a promotion series. A division promotion series means you have to win two out of your next three matches to get promoted, while in a tier promotion series you have to win three out of five.
Once you enter a tier, you can not drop from it by losing games -- but you can drop divisions. So if you are Gold I you could potentially drop to Gold II, but as long as you are active you will never get demoted to Silver tier.
As mentioned earlier, you can lose League Points by losing games, and this point loss will drop you divisions but NOT tiers when you go below 0.
But, when you're inactive for 28 days, your League Points start decaying. This point loss will drop both divisions AND tiers -- again, when you go below 0.
The decay rates are set by tier:
- Bronze: No weekly decay (I assume this means you will stay in Bronze 1 unless you decide to lose some games)
- Silver: 10 per week
- Gold: 25 per week
- Platinum: 35 per week
- Diamond: 50 per week
The challenger tier is different - it has no divisions, and league points have no upper bound. Only 50 players and 32 teams (correct me if I'm wrong on the solo count) are allowed in there, and when a new challenger enters the tier, the person with the least league points will get demoted to Diamond I (the top division of the otherwise top tier).
Inactivity starts after 7 days, not 28.
When someone completes their Promotion series for Challenger, the lowest player / team in Challenger is Bumped and dropped to Diamond I. This seems to be in order to keep the challenger as a list of the best current players in League of Legends.
The priority list for bumping out of Challenger:
- If any player / team is inactive (remember, 7 days), only an inactive player / team can be bumped.
- The player / team with the lowest league points will be bumped.
- (Unknown behavior to resolve LP ties)
To summarize:
- Can you drop tiers other than Challenger?
- Can you drop divisions?
- Can you drop Challenger tier?
- Yes, by Decay OR Losing OR Bumped.
Best Answer
Master and Challenger Tiers work a bit different from the others (Bronze-Diamond).
First of all there is only 1 division in both but there is no upper limit in LP. Also the promotion from Master to Challenger happens automatically. Every 24 hours the MMR is checked and as soon as someone in Master has a higher MMR than a certain Challenger player he will take his spot.
In Master there is no player limitation but the Challenger tier is limited to 200 players.
Also in both of these special tiers LP decay happens a lot faster than in Platinum/Diamond. In Plat/Dia LP decay starts after 28 days of inactivity and drains LP every 7 days after while in Master/Challenger LP decay starts after only 10 days and drains LP every day after.
You can read more in the Riotgames Ranked-Play FAQ
Reaching master or even challenger proves that you are amongst the absolute elite in your current region. The skill difference between Challenger and Diamond 2 is usually bigger than the difference between Bronze 5 and Diamond 2 because it requires you to master absolutely every aspect of the game (or your main champion).