Massive health stacking and tower smashing.
Durability and crowd control
Every 4s your next attack on a champion deals bonus damage, heals you, and permanently increases your health.
Charge up a powerful attack against a tower while near it.
After 10 minutes gain 8 Armor and 8 Magic Resist and increase your total Armor and Magic Resist by 5%.
Every 8 monsters or enemy minions that die near you permanently increases your max health by 0.2%.
+65 health and +10 AP or 6 AD, adaptive
Periodically your next ability used has its mana or energy cost refunded and restores some of your missing mana or energy.
Your first ability hit every 20s burns champions.
Sion's natural % health stacking on [W] synergizes with the oh-hit health stacking on Grasp letting him hit massive health numbers in the late-game.
With Demolish we can convert the massive health stackng on [W], Grasp, and Overgrowth to big-time tower damage.
If you're looking to teamfight more often then Font of Life is a good alternative in healing your teammates.
The Armor and Magic Resist you gain from Conditioning are even more effective because you are stacking so heavily on health.
Alternatively you can take Iron Skin or Mirror Shell here instead, based on the enemy composition.
Overgrowth's bonus health scaling synergizes incredibly well with Sion's [W] passive making you a huge front-line tank with massive amounts of health.
Manaflow Ban's massive mana sustain allows you to use your large health pool to widdle away at your opponent's health bar in lane and out-sustian them.
In difficult AP matchups (Rumble) or when the enemy team has a lot of magic damage (Kog'Maw), Nullifying Orb can be a great alternative to help you absorb even more damage in the front-line.
Scorch's extra bit of lane harass damage allows you to take a lot of trades in the laning phase with long-range [E]'s and come out ahead of your opponent.