Spell Conversion: Bigby’s Besieging Bolt in 5e

In our Spell Conversion Series we will look at spells from 2nd edition that didn’t make it into the 5th edition of D&D. For a full list of all named spells, go here.


In order to convert a spell we need to look at the guidelines of the Dungeon Master’s Guide, at the original spell in 2nd edition and at the existing spells in 5e. The new spell shouldn’t overshadow an existing spell nor be too complicated.

5th edition’s biggest advantage over older editions is that spells are extremely streamlined and bookkeeping on the Dungeon Master’s side is kept to a minimum.

Also, some spells might be used for a specific situation. This showcases how some mages in Greyhawk invented magic for their own needs. Consider players to get in touch with those unique spells by meeting disciples of these famous mages (or even the mage himself) rather than handing them out freely.

The spell

Today we look at Bigby’s Besieging Bolt. The spell mimicks a big siege weapon. Sieging weapons are especially used in war scenarios. As wars are normally not the main topics of campaigns, it is understandable that it was left out.

But the spell is still unique and can be converted. Let’s look at the important mechanics we want to convert:

  • 6th Level, one target
  • Never misses
  • Mimicks a siege weapon

The spell is best compared with being a mix between arcane missiles and standard siege weapons. In the original the caster would have to wait a turn between shots. This is not a fun mechanic for a player to be forced to sit out turns, so we get rid of this mechanic.

Damagewise this spell shouldn’t exceed the Magic Missile damage against creatures. Magic Missile does 9d4 + 9 (31), 10d4 + 10 (35) and 11d4 + 11 (38) damage on levels 6, 7 and 8, respectively.

Ultimately, the spell should be used for almost exclusively for structure damage.

Considering all of this, this is my conversion of Bigby’s Besieging Bolt in 5e:

Bigby’s Besieging Bolt

6th-level evocation
Casting time: 1 action
Range: 480 feet
Components: V, S, M (small stone sprinkled with diamond dust)
Duration: Instantaneous

You create a one magical siege bolt mimicking a Ballista. You hit the target with a magical bolt doing 3d10 force damage. If the target is structure, the damage is doubled.

At Higher Levels. On 7th/8th level the spell mimicks a Mangonel/Trebuchet doing 5d10/8d10 force damage (doubled against structures) and has 800/1,200 ft. range (can’t hit targets


As you might have realised, converting spells is never easy and highly subjective. There are many ways to the same goal. Our goal is to make our World of Greyhawk experience unique, different from a homebrew of Forgotten Realms campaign. And with every new spell that used to be a Greyhawk exclusive, we are getting closer to our goal.

Next time we are going to convert Bigby’s Bookworm Bane.

See you in the Green Dragon Inn,


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