Selena: Best build, Counters

Symbiosis

Selena can switch freely between her two forms.
Elven Form: Selena applies Abyssal Mark when her skills deal damage. Marks last 8s and can stack up to 2 times.
Abyssal Form: Selena's Basic Attacks become melee and deal (+25% Total Magic Power) additional Magic Damage. Her skills deal 240-450 (+40% Total Magic Power) additional Magic Damage to enemies with Abyssal Mark and consume their mark stacks by 1. This damage is increased to 250% against Creeps and Minions.

Abyssal Trap

CD: 8 Mana Cost: 70

Selena summons an Abyssal Devil to lurk underground. The Abyssal Devil will attach itself to any enemy that approaches, inflicting a stack of Abyssal Mark and reducing their Movement Speed by 50% for 1s. The Abyssal Devil then explodes, dealing 350 (+150% Total Magic Power) Magic Damage to enemies in the area and reducing their Movement Speed by 50% for 1s.
Each additional Abyssal Devil on an enemy increases the final damage by 50%.
Each Abyssal Devil can lurk up to 60s and up to 3 Abyssal Devils can exist at the same time.

Abyssal Arrow

CD: 12 Mana Cost: 80

Selena fires an abyssal wraith in the target direction that stops when it hits an enemy hero, Creep, or Summon, dealing 250 (+20% Total Magic Power) to 500 (+40% Total Magic Power) Magic Damage to the target hit and stunning them for 0.5s to 2.5s while revealing their locations (the damage and stun duration increase with the wraith's travel distance).
The wraith will pick up Abyssal Trap as it travels over it and will apply the trap to the enemy target upon stunning them.
If the wraith stuns an enemy for 0.9s or longer, Selena will gain 60% Movement Speed for 2s.

Primal Darkness

CD: 5.5

Passive: Selena gains 5 Magic Power.
Active: Selena enters Abyssal Form and gains 20% Movement Speed for 0.8s. This also immediately resets the cooldown of Soul Eater and Garotte.