Making 5E Monsters on the Fly, Fiction-First; the DMG in Your Head

This will let you improvise creature stats on the fly.

NOTE: These are not meant to approximate or reproduce any existing statblocks. Creatures done this way *I think* will be tougher than those of equivalent HD in the official stuff. They'll definitely have more hp and will likely have higher to-hit values. Based on my playing of 5e so far, that's a good thing. PCs getting too cocky these days.

I HAVE NO IDEA IF IT IS "CORRECT", but it seems fine from where I'm sitting. There is no warranty.

HD type: by size

• d4: tiny
• d6: small
• d8: medium
• d10: large (+1 damage dice, +2 damage)
• d12: huge (+2 damage dice, +4 damage)
• d20: gargantuan (+4 damage dice, +8 damage)

Example: a hobgoblin may do 1d8 + 4 damage with its Morningstar. If it were made huge (giant size), it would gain +2d to damage and +4 flat on top, for 3d8+8. 

HD value: don't roll; just max it

# of HD: assign tier first (Basic / Expert / Champion / Master / Immortal), then number of HD within that range by combat role or by scariness:

• caster: least HD / scary 
• mixed caster: next least HD / scary
• martial: next most HD / scary
• tank: most HD / scary

Assign proficiency and gear by what seems legit; don't worry about what's permitted; worry about what seems true.

Instead of additional attacks, casters have spell slots.

A caster can use all attacks for spell slots and still have 1 physical attack left, but it's offset by not being proficient with good weapons and armors.

Assign spells to casters by what seems legit. 

Stats by HD range

• HD 1-4 (Basic); +4 d+2 x 1; ST 2/0/-2; DC 12
• HD 5-8 (Expert); +6 d+3 x 2; ST 3/1/-1; DC 14
• HD 9-12 (Champion); +8 d+4 x 3; ST 4/2/0; DC 16
• HD 13-16 (Master); +10 d+5 x 4; ST 5/3/1; DC 18
• HD 17-20 (Immortal); +12 d+6 x 5; ST 6/4/2; DC 20

How to read those statblocks:

• HD X-Y: how many hid dice the creature has, corresponding to its tier
• +X (attack) d+Y (damage bonus) x Z (number of attacks or spell slots or combination thereof)
• ST X/Y/Z (saving throw bonuses: good/okay/bad
• DC X: difficulty class of saving throws 

Example

A cursed knight, very scary. 

I use the expert tier cause she's serious business but still possible for a starting party to take on if lucky. Since she's very scary, I use the most HD possible from the expert tier (HD 8). So that makes her:

• AC 20 (plate plus shield)
• HD 8d8 (64 hp)
• Attack +6 for 1d8+3 damage, two attacks
• Saves: Strength and Dex: +3, Wis -1; else +1
• Save DC 14 (say for martial effects, like being tripped)