Living Bow
Requires Attunement
Ranged weapon (martial, bow)
Damage: 2d8
Damage Type: Piercing
Properties: Ammunition, Range, Two-Handed
Range: 300/600
Weight: 4
The Living Bow resembles a tree branch wrapped in vines more than a bow. It will string itself and when drawn, create its own arrows, also made of living wood.
When the damage dice of the Living Bow roll doubles, the Living Bow can cast a spell corresponding to the rolled value. The save DC of this spell is 12 plus the wielder's proficiency bonus, its attack bonus is 4 plus the wielder's proficiency bonus, and the Living Bow can only maintain concentration on one such effect at a time. The spell is cast on the target regardless of range requirements.
1: Thorn Whip (damage dice rolled is based on wielder's level)
2: Entangle (centered on target)
3: Spike Growth (centered on target)
4: Grasping Vine (adjacent to the target)
5: Roll 1d4 and use that effect.
6: Pick any effect from 1 through 4.
7: Roll 1d4 twice, and trigger both effects (even if both require concentration).
8: Wrath of Nature (centered on target)
Once you have used one of these abilities (from 1 to 8) you cannot use it again until you complete a short or long rest. If the spell allows you to take further actions to control the spell, you can do so while wielding the bow.
In the event of a critical hit, if more than one pair is rolled choose one pair to trigger.
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Design points:
1: +X items are powerful but boring in comparison to the power level, especially accuracy buffs.
2: No need to exclude smols from this weapon. So no Heavy keyword.
3: Offensive abilities on a weapon are good.
4: Minigames where the actual pip values on dice matter are good. But they have to come up reasonably often; not too often or too rarely.
5: This weapon should lean into the "living plant material" vibe.
At 2d8 damage it is tempting to use even without the spells: as +2 damage is worth about +1 accuracy, the +3.5 damage on this weapon over a longbow makes it on-par with a +2 longbow.
The spells trigger 1 in 8 attacks (~13%) and ~60% of crits (100%-8Choose4/8^4), enough to be happening often you won't forget about it, but not every turn. The "once per rest" restriction keeps it from getting ridiculous, and makes "spamming it out of combat" less silly.
I also gave it a 300 short range. This is an accuracy boost, but not a numerical one.
And the fact that most crits give you a spell effect is a fun side effect of the "any pair of dice" mechanic.