Ambush. If the target of a heart seeker bow attack is suprised or if the Echidna has advantage on the attack roll. The attack deals an additional 21 (6d6) poison damage.

Blessing of the Death God (3/day). The Echidna captain can bless a creature within 60 ft with Myrkuls power. The creature goes into an enraged state and their next attack has advantage and deals an additional 18(4d8) necrotic damage, then they explode in an eruption of gore and dark magic dealing 22 (4d10) necrotic damage to all creatures within 15 ft. Or half as much on a successful DC 16 constitution save.

Innate Spell Casting. The Echidna is an inate magic user. Wisdom is her spell casting ability (DC 17 with a +9 on attack rolls) she has the following spells at her disposal.

At will: inflict wounds, false life, guiding bolt (necrotic) 

(3/day): lightning bolt, symbol, invisibility

(1/day): chain lightning, harm

Multiattack. The Echidna makes either three melee attacks with her death glaive--or two ranged attacks with her heartseeker longbow.

Death Glaive. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 16 (2d10 + 5) slashing damage plus 14 (4d6) necrotic damage. If this weapon reduces a creature to 0 hit points they must make a DC 17 constitution save or die.

Heartseeker Longbow. Ranged Weapon Attack: +11 to hit, range 150/600 ft., one target. Hit: 14 (2d8 + 5) piercing damage then must make a constitution saving throw. On a failure the target takes an additional 18 (4d8) poison damage and becomes poisoned. 

Death Chant (recharge 5-6). Target creature within 60 ft must make a DC 17 constitution save. On a failure they take 14 (4d6) necrotic damage. They gain vulnerability to the next attack that hits them. If that attack reduces them to 0 they die.

Spacing technique. Whenever a creature moves into range the Echidna can use her reaction to make a melee attack against the creature. If the attack hits the Echidna moves half its speed away from them. This movement doesnt provoke opportunity attacks.