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.
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 unless the saving throw roll would have succeeded on a DC 12 Dexterity save3.
Effects on Objects
Objects in the path of the stampede take 5d8 bludgeoning damage. Objects 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 has to make a saving throw, but the spell's bludgeoning damage is reduced by half.
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.
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.
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?
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. Some spells do have multiple save chances, so an alternative would be that the second roll is DC 12 Constitution or Dexterity to avoid trample damage, or to use "if the saving throw result is 6 or lower, take the additional damage" which is similar to how the Sprite's put them to sleep ability works.