Rell Runes explained
There's one set of runes for Rell on patch
11.8
that is the clear favorite for LoL players and pros alike. The best Rell Runes on the position 
Support are Resolve as the primary path and
Inspiration as the secondary path with a pick rate of 100%. Besides runes you should take a look on Rell Build and if you are interested in pro matches take a look at Rell Probuilds. If you want to discuss
Rell then you should visit the champion subreddit.
Primary path on Rell
Resolve empowers the survivability of Rell and increases the tankiness. Most of high elo players – and our LoL pros in the the major regions – have agreed that
Aftershock is the best keystone rune to run on
Rell. This keystone rune bulks you up, increasing both armour and magic resistance, once you immobilized a champion.
Aftershock synergises both with the sheer amount of crowd control Rell possesses and the need to be tanky as soon as you engage. This is usually followed up by taking
Shield Bash,
Bone Plating and
Unflinching.
Shield Bash powers up your tankiness and empowers your next basic attack while you have a shield.
Bone Plating make Rell overall tankier, by decreasing the amount of poke damage in the early game.
Unflinching inceases Rell tenacity based on your missing health.
Secondary path on Rell
When it comes to your secondary path, most
Rell players prefer to go with
Inspiration.
Inspiration opens creative opportunities on Rell troughout the match. On the
Inspiration path, the best runes for Rell are
Hextech Flashtraption and
Cosmic Insight.
Having an extra way to decrease the distance with the
Hextech Flashtraption will make an engage much easier to pull off.
Cosmic Insight grants summoner spell and item haste.