When using hail of thorns is the spell attack bonus added to the roll?
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No. On the first successful hit (using a normal weapon attack), Hail of Thorns forces the targets within 5' to make a Dexterity save against the spellcaster's spell DC or else take full damage (half damage on a miss). The spell DC for the character's spells is calculating similarly, but separately form the character's spell attack bonus.
Spell casting Ability (Ranger)
Wisdom is your spellcasting ability for your ranger spells, since your magic draws on your attunement to nature. You use your Wisdom whenever a spell refers to your spellcasting ability. In addition, you use your Wisdom modifier when setting the saving throw DC for a ranger spell you cast and when making an attack roll with one.
Spell save DC = 8 + your proficiency bonus + your Wisdom modifier
Spell attack modifier = your proficiency bonus + your Wisdom modifier