Stampede: How does this custom spell compare to Cloudkill, another 5th level spell?

Question

Is this stampede spell (a proposed custom spell for Druids and Rangers in my upcoming campaign) as strong as, or weaker than, the 5th level spell cloudkill? I used a similar base damage, the duration is reduced from 10 minutes to 1 minute, and the "persistent" effect of the poison is replaced by a chance to be trampled if knocked prone. I reduced the directional control somewhat.

Stampede
Spell level 5 (conjuration)
Casting Time: 1 action
Range: 120 feet
Components: V, S Duration: Concentration, up to 1 minute

You create a 40' wide, 20'deep, line of spiritual herd animals¹ (buffalo, horses, giraffes, zebras, etc) at a point you choose within range. The animals stampede in the direction that you choose and will continue in that direction until either the spell ends or you cease concentration on it. The stampede moves at a speed of 40'.
Effects on Creatures
Any creature in the spell’s area of effect on the caster's turn takes 5d8 bludgeoning damage on a failed Dexterity saving throw, and is knocked prone. On a successful save, the creature takes half damage and is not knocked prone. Any creature knocked prone takes an additional 2d4+3 bludgeoning² damage if the saving throw result was 11 or lower³.
Effects on Objects
Objects that are not held or worn by creatures in the path of the stampede take 5d8 bludgeoning damage. Objects of size Large, or larger, that are not destroyed by the bludgeoning damage cause the stampede to flow around the object. Any creature caught in the area of that flow around an object takes half damage on a failed Dexterity saving throw, and one quarter damage on a successful one.
Effects of Terrain on the Spell
The herd will plunge through rivers, go over cliffs, or try to flow around a cliff base's face (likewise with castle walls that are not knocked down, gigantic rock outcroppings, etc). Difficult terrain will not reduce the speed of the stampede. This spell can be used underwater, and will manifest as herds of spiritual giant sea horses, schools of large fish, etc.

At Higher Levels. When you cast this spell using a spell slot of 6th level or higher, the damage increases by 1d8 for each slot level above 5th.

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In trying to get the flavor right, I am not sure if the relative power level of the spell is retained at 5th level. Is this comparably powered to cloudkill at 5th level?

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¹ These herd animals are similar to what is summoned by spirit guardians, rather than creatures that are summoned by the conjure animals (Druid, level 3)(Response to @MikeQ). There isn't a stat block for a herd in the way that there is for a swarm.

² This is based on MM/PHB riding horse hoof damage.

³ The fiddly bit was intended to avoid making two saving throws; one roll covers it all. Some spells do have multiple save chances, so an alternative would be that the second (Constitution or Dexterity) to avoid trample damage. I modeled this after the Sprite's "if the saving throw result is X or lower, it puts them them to sleep" ability.

Answer 1

It is ok for level 5

Let's compare all aspects of the spells

Damage type

Poison is the worst, while magical bludgeoning is the best.
Stampede wins by a large margin.

Range

Same initially, Stampede can reach much further in the end.
Stampede wins.

Area and damage

These are hard to separate.
Stampede does more damage per round, if the target falls prone, but it can only damage an enemy multiple times if it runs in the direction the charging animals do, and about the same speed, meaning away from you. If they are running away, you already won.
Cloudkill moves much slower, a stationary enemy will remain in the area for several rounds.
Cloudkill wins by a very large margin against most enemies.

Duration

Most encounters do not reach the fifth round, and on average are about 3 rounds. So both will last the whole combat.

In my experience combat encounters are mostly further apart in time than 10 minutes, closer to 60. If they are more frequent, it usually happens because the party is running somewhere. These two together mean that area spells most of the time last a single encounter, regardless of duration.

About as good as Cloudkill

Unless I know that the Second Goblin Army is coming, I will prepare Cloudkill. It has a horrible damage type, but I will simply use something else against undead and elementals.
But at least it has a quite good chance to damage my enemies more than once.