TLDR:
1. You need Sweet Dreams Legendary Shard
2. Nightmare cooldown 6 seconds or less, nightmare invulnerability 3 seconds or more (4 elite shards or 7 normal shards)
3. Enough mana regeneration
You need this legendary shard:
Sweet Dreams: Friendly Nightmared units gain a 200% longer invulnerability period that grants them 750 unslowable movespeed with flying, and any Brain Saps occuring while an ally is Nightmared will heal it.
Common shard you need to take:
1. Nightmare friendly invulnerability: 1 sec base invulnerability, a common shard will give extra +0.5 sec (+1 sec if elite) invulnerability.
2. Nightmare cooldown/mana cost reduction: with level 20 talent and octarine core, your nightmare cooldown will be 7.5 seconds. Common shard will reduce the cooldown by 7% (14% if elite).
The 7% reduction will be around 7.5 - 7.0 - 6.5 - 6.0
The 14% reduction will be around 7.5 - 6.5 - 5.5
So you need 3 common shards, 2 elite shards, or 1 elite shard and 1 common shard for cooldown/mana cost reduction and you need 4 common shards or 2 elite shards to make friendly invulnerability to 3 sec.
In a perfect run you need 4 elite shards for this to happen, upgrade shard blessing is a very helpful blessing if you play a lot of this game. Don't forget to check the Oracle to increase your chance.
You want to make sure the cooldown is less than 2x nightmare invulnerability. For example cooldown 6 sec, nightmare invulnerability 3 sec.
Sweet Dreams legendary shard increase the invulnerability by 200% so 3 sec means 6 sec. You also want to have good mana regen items.
Items:
Octarine Core
Bloodstone: mana regen and survivability
Wind Waker: survivability and mana regen
Radiance: extra damage with no mana
Optional damage boost:
Dagon
Meteor Hammer
Ethereal Blade
Veil of Discord
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