[RPG] What level spell is Erupting Earth from Elemental Evil

When you cast this spell using a spell slot of 3rd level or higher, the damage increases by 1d8 for every two slot levels above 2nd.

Spritual weapon is a 2nd level spell. The first part of this description is simply telling you the rules for casting it from a slot higher than it's level, i.e., third level or higher. Many spells have greater effects when they are cast with a higher level spell slot.

The second part tells you specifically what effect casting with a higher slot has and how the spell improves based on which spell slot you use. In other words, casting it from a 3rd level slot doesn't grant any benefit, because that is not two slot levels above second.

The spiritual weapon's damage works out as follows:

• 2nd level slot: 1d8 + your spellcasting ability modifier.
• 3rd level slot: 1d8 + your spellcasting ability modifier.
• 4th level slot: 2d8 + your spellcasting ability modifier.
• 5th level slot: 2d8 + your spellcasting ability modifier.
• 6th level slot: 3d8 + your spellcasting ability modifier.
• 7th level slot: 3d8 + your spellcasting ability modifier.
• 8th level slot: 4d8 + your spellcasting ability modifier.
• 9th level slot: 4d8 + your spellcasting ability modifier.

The 6d10 maximum damage is not relevant for an usual ranger, but is brought into play when casting the spell at higher levels than five, which can happen...

By being a Bard

The Bard has spell slots all way to ninth level, and the class feature magical secrets allows them to learn spells of any class. A Bard could cast the spell with a sixth-level slot or above, dealing 6d10 damage.

By multiclassing into a spellcaster

If a Ranger takes the spell early, they can multiclass into Wizard, Sorcerer, Bard, Cleric or Druid and eventually gain access to sixth-level and higher level slots and use them to cast the spell at higher-than-usual levels.

By casting Wish to duplicate the effects of Hail of Thorns

A high-level caster can use the spell Wish to duplicate the effects of any lower-level spell, including Hail of Thorns. In this case, it'd be cast as a ninth level spell as described here.