Restore health over time based on your missing health.
Arcane Comet on Sion in an incredibly reliable source of damage thanks to his natural slows on [Q] and [E]. In the laning phase, paired with Scorch, Comet provides fantastic lane pressure damage when maxing [E] first.
Manaflow Band is your means to mana sustain in the laning phase so that your capable of continuously harassing with [E] and comet.
Transcendence is a great rune choice on tanks who are looking to stack a lot of big CDR items and can still benefit massivly from the extra bit of AD or AP after capping.
Sion loves the extra bit of AD.
Scorch is your go-to lane harassment rune. Combined with [E] max and Comet you'll be able to safely harass from a distance when passing [E] through minions.
Bone Plating's early game damage mitigation makes your early lane trading incredibly effective thanks to Sion's high base damage and CC.
Second Wind paired with Iron Skin allows you to take a number of early trades and come out with a health lead.