Explanation behind Sion's title, the Undead Juggernaut

A tank has heavy lockdown and durability to continue locking down enemy champions. Juggernaut has high damage, little to some mobility (but usually no dashes), and durability through in-combat sustain.

When Sion is alive, he's a tank. His Q, E, and R are all disable/CCs. His W is a shield to increase his durability, as well as provide some damage. When Sion is killed, he becomes a juggernaut. Sion's abilities/base stats are changed so that he gains massive amounts of lifesteal and on-hit damage, ton of attack speed, and some movement speed.

So there, that's why Sion is the Undead "Juggernaut."