How does rune combiner work?

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.

As answered by Raven it all comes down to chance. I myself found if you do this at the start when your level 1 you gain a tremendous advantage over the rest of the players until later level.

The reason for this is that the Tier 2 and Tier 3 runes have a level requirement you have to reach in order to purchase them from the shop. If you use the Rune Combiner you can get the better Tier 2 to Tier 3 runes much sooner and actually place them into your rune page. Early on just hold onto the good Tier 2's you get and use them but hold onto the ones you won't use until you save up 5 of them for making the Tier 3 runes you'll always use later on - just buy the cheapest Tier 1 runes from the shop early (duplicates don't matter, just the absolute cheapest Tier 1's in the shop) and use 5 of them to create a Tier 2 rune.

For the ones you don't see yourself using save them until you have enough to create a Tier 3. Continue doing this until you reach the level requirement to buy the Tier 3 runes straight from the shop.

By this point you should have a nice amount of Tier 3 runes that will aid you tremendously. My last tip is that you should stop using the Rune Combiner when you reach around 40% maximum completion of having every single rune with the maximum set amount.

I found if you try to go push beyond that point you waste IP because you get Tier 3 duplicates which do you no good at all.