So we have 4 playable Darkin in league right now and they all fill in each role except support. Aatrox is top, Rhaast is jungle, Naafiri is mid, and Varus is adc. If we add in Joraal, the Aegis of Pain, we can have a full team of Darkin, a Darkin skin line, expansion on Darkin lore and the Great Darkin War, special interactions with other Darkin and Void creatures, etc. I know there are other Darkin from LOR that could be interesting picks but Joraal is a PERFECT support pick. I came up with a rough draft for his kit and is subject to change based on ideas:
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Joraal Abilities:

Passive: Attacking an enemy with an ability marks the champion for 5 sec. Any damage dealt to the marked champion heals joraal for % of his max health. Halfed for damage done by Joraal.

Q: Joraal becomes unstoppable and increases movement speed for a small duration. Upon recast he leaps a small distance and slams his sword in the ground creating difficult terrain (grounded) and slowing.

W: Joraal radiates an aura that causes enemy mana costs to go up. Upon an enemy using a spell it zaps them for a percentage of their maximum mana and slows. (Ionic Spark)

E: Joraal shrieks out in agony and despair causing nearby enemies to be slowed with the slow increasing based on how close you are to Joraal.

R: Joraal heals allies in an area based off damage he receives. Heal is increased based on number of allies in vicinity. He also reflects a small portion of all damage received.
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There are no specifics on numbers due to this just being a rough draft. You may also note there isn’t any hard CC, this is because I believe there is just too much CC in League atm. Joraal would focus on slows and movement prevention. I tried to keep his kit loyal to his lore and card info from LOR, as well as unique enough to where it isn’t a copy paste of anyone else’s abilities especially Leona. Joraal pics for reference:

Legends of Runeterra Card

Joraal \“Legends of Runeterra\” Concept (by Riot Artist Aron Elekes)