Level 1 / Tier 1
Block : Every 7 seconds, gain 50 Physical Armor against the next enemy Hero Basic Attack, reducing damage taken by 50%. Stores up to 2 charges.
Dark Skin : Every 7 seconds, gain 50 Magical Armor against the next enemy Hero spell, reducing damage taken by 50%. Stores up to 2 charges.
Demon Blood : Q : Gathering 30 Regeneration Globe increases your Health regeneration by 1 per second, up to 30.
R : After gathering 30 Regeneration Globe, your demon skin is empowered and grants you 500 Health.
Level 4 / Tier 2
Blood Lust : Any enemy hit by Slash recovers you 5 mana.
Sharp Spear : Slash makes enemies bled for 25% of its dealt damage.
Demon Rage : Increase the amount of damage dealt by Cleaving Attack to 75%.
Passive : Increase your attack-speed by 25%.
Level 7 / Tier 3
Masochism : Increases regeneration effects and all healing received by 30%.
Sadochism : Using Slash on enemy Heroes heals you for 30% of its damage.
Quick Combat : Basic Attacks reduce the cooldown of Slash by 0.25 seconds, up to 0.75 seconds with Cleaving Attack.
Level 10 / Tier 4
Fel Spike : Mannoroth hurls a spike on the ground and cover the area of fel energy which deals damage to any enemies. It deals 70 (+4% per level) per second until Mannoroth or any enemy break it.
Infuse : Mannoroth infuses his blood in his minions' veins and every fountains on the battlefield. Mannoroth grants 25% bonus health and damage to every minions in his team.
Active : Mannoroth yells and drives his hordes in a frenzy rage. Minions have 30% bonus attack-speed for 10 seconds.
Passive : Decrease health regeneration by 5.
Passive : Decrease amount of healing given by enemy fountains by 40%.
Level 13 / Tier 5
Burn it : Deal 23 (+4% per level) damage per second to nearby enemies.
You, Fiends : Using Slash on minions deal 50% more damage.
Slaves shall serve : Using Slash on Heroes deal 75% more damage.
Level 16 / Tier 6
Fel Infused Blade : Attacking a hero that is slowed, rooted or stunned increases Mannoroth's next Basic attack damage by 70% for 3 seconds.
Grievous Rage : Using Slash empowers the next Cleaving Attack by 50%.
Stomp : Decrease by 3 seconds War Stomp cooldown, per hero hit.
Level 20 / Tier 7
Fel Infused Spike : The Spike is empowered by fel and deals 170 (+4% per level) and slows for 30% in a small area.
Greater Infuse : Allied heroes shares Mannoroth's blood and it boosts them.
Active : Empowers allied health regenration by 10 and their attack-speed by 20%.
Empowered Demon Skin : Activate to have a shield equal to half of Mannoroth's current health. Bonus given by Demon Skin are inactive during the cooldown. ( 60 seconds cooldown. )
Nexus Blades : Basic Attacks deal 20% more damage and slow enemy Movement Speed by 20% for 1 second.
Heroes of the Storm Build Guides
Overall, Great concept, would definitely want to see in the game, but I don't think he would pass for a specialist. I personally think he would pass as a Warrior more than anything.
As for the assassin, none of his talents really make him "assassin-y". He has some damaging talents but he still lacks a lot of DPS. If you have your heart set on him at least being a Warrior/Assassin, I would give him some more % health damage talents and call it a day.
I'd also redo his E because it seems a bit boring. Maybe replace it with an ability that places down a mini healing 1-use healing fountain that works as a regen globe but doesn't apply to talents or something else to give him that little bit of specialist? Or possibly redo his passive to something like ETC but its attack speed or attack damage and redo his second ult?