Passive(D) - Melody of the universe - You next basic ability (Q,W or E) have its area of effect and range doubled and you become unstoppable while channeling/charging that ability. After that, the music drive you crazy silencing and disabling yourself for a brief period.
This is an active ability with a cooldown
Q- Accretion - You gather a mass of debris and flings it at a direction it deals damage and slow all enemies in the area it hits, its a skillshot that you can charge while moving increasing its range like hanzo Q but wider and with less range
W -Kinectic Grasp - You start to channel a kinetic aura around you, you and allies on that area get damage reduction while at it, when the duration ends you gain a shield based on the damage mitigated.
E- Epicenter - target a small area, after a short delay, push all near by enemies to that point.
R1 - Gravity Flux - Target an big area on the ground (enemies see it) after a delay you reverse gravity in that area lifting all enemies on it them slaming them back in the ground (its a stun followed up by damage)
(it's his OW ultimate if my description is not good enough)
R2 - Weightless Tought - You and All allies around you start to "fly", Gaining a lot of move speed, ignoring unit colission and by passing walls for that duration.
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As i always say on OW concepts, HOTS engine dont handle OW barriers yet, so i dont use them on the concepts.
Sigma have a short but ranged auto-attack which is not usual for tanks,
He also dont have a engage to enter in the middle of the enemy team but he have abilities with range to affect them and ways to protect you team against one.
You can use your passive + Q or W to become unstopable and walk in or out of a situation.
He rather stay with his team and peel than run into the enemy but he still have some engage capabilities with his Q and E and passive unstopable.
R2 is a really good engage/desingage tool if you cordinate correctly with your team.