In 2015 the angry GM wrote a thing about combining monsters to create bosses. It’s a good article! But he makes a weird assumption at the start. He acknowledges that 5e fixes the “single monsters are weak” problem with Legendary Actions but then claims those are too awkward and creates his own way of making a fight with a group of monsters seem like it’s a fight against a single monster. 

His system is pretty cool and can make for interesting fights with arbitrary amounts of baddies stacked on top of each other. I recreated the wolf-in-sheep’s-clothing from Expedition to the Barrier Peaks in The Sunken Ruins of Ishau by stitching together a gibbering mouther and some giant constrictor snakes. But I’m not convinced that the boys at WotC hadn’t thought of all this themselves. In fact I think a creature with legendary actions is exactly that: two creatures pretending to be a single boss.

How Strong Are Two Snakes?

So here’s the idea: I want to create a battle with a big bad boa constrictor boss. But when designing combats in 5e it’s best to have a group of enemies. Can I get the best of both worlds? 

Well sure, just have a fight against two giant constrictors.

The CR for a giant constrictor snake is 2 (450 XP). A fight against two of them is worth 1350 XP, because of the 150% multiplier (see “Creating a Combat Encounter” in the DMG). That’s about the same as a single CR 4 monster (1100 XP).

I’ve done this calculation for each CR and wrote it in a handy table below. I’ve also added the party level according to the Encounter Building in Xanathar’s Guide to Everything.

CR	Boss CR	Player lvl (party of 4)
1/8	1/4	1
1/4	1/2	1
1/2	1	1
1	2	1-2
2	4	2-4
3	5	3-4
4	7	4-5
5	9	5-7
6	10	6-8
7	12	9-11
8	14	10-11
9	16	11-13
10	16	11-13
11	18	13-15
12	20	15-17
13	20	15-18
14	21	16-19
15	21	16-19
16	22	17-20
17	23	19-20
18	23	19-20
19	24	20+
20	25	20+
21	26	20+
22	28	20+
23	29	20+
24	30	20+

Combining Two Snakes

Two snakes have twice the amount of hit points as one snake has. But they also lose half of their effectiveness once they reach half health (assuming the players focus their fire on one snake). We can simulate that by giving the combined snake 150% of a snake’s regular hit points.

Two snakes attack twice as often as a single snake. We could give our snake multi-attack, but it’s nicer to spread the extra attacks out a bit using legendary actions.

Two snakes are harder to take out of a fight with spells like sleep or polymorph than one snake. Legendary resistances allow the single snake to compensate.

Stats for Two Snakes

Two Snakes

Huge beast, unaligned
Armor Class: 12
Hit Points: 90
Speed: 30 ft., swim 30 ft.
Challenge Rating: 4 (1100 XP)

STR	DEX	CON	INT	WIS	CHA
19 (+4)	14 (+2)	12 (+1)	1 (-5)	10 (+0)	3 (-4)

Skills: Perception +2
Senses: blindsight 10 ft., passive Perception 12

Languages: —

Legendary Resistance (3/Day). If the snake fails a saving throw, it can choose to succeed instead.

Actions

Bite. Melee Weapon Attack: +6 to hit, reach 10 ft., one creature. Hit: 11 (2d6 + 4) piercing damage.

Constrict. Melee Weapon Attack: +6 to hit, reach 5 ft., one creature. Hit: 13 (2d8 + 4) bludgeoning damage, and the target is grappled (escape DC 16). Until this grapple ends, the creature is restrained. The snake can grapple two targets at a time.

Legendary Actions

The snake can take 1 legendary action, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature’s turn. The snake regains spent legendary actions at the start of its turn.

Bite. The snake makes a bite attack.

Grapple. The snake attempts to grapple a target.

Move. The snake moves up to its movement speed without triggering Opportunity Attacks.

Another Example: Greater Medusa

Greater Medusa

Medium monstrosity, lawful evil
Armor Class: 15 (natural armor)
Hit Points: 205
Speed: 30 ft.
Challenge Rating: 10

STR	DEX	CON	INT	WIS	CHA
10 (+0)	15 (+2)	16 (+3)	12 (+1)	13 (+1)	15 (+2)

Skills: Deception +5, Insight +4, Perception +4, Stealth +5
Senses: darkvision 60 ft., passive Perception 14
Languages: Common

Legendary Resistance (3/Day). If the medusa fails a saving throw, it can choose to succeed instead.

Petrifying Gaze. When a creature that can see the medusa’s eyes starts its turn within 30 feet of the medusa, the medusa can force it to make a DC 14 Constitution saving throw if the medusa isn’t incapacitated and can see the creature. If the saving throw fails by 5 or more, the creature is instantly petrified. Otherwise, a creature that fails the save begins to turn to stone and is restrained. The restrained creature must repeat the saving throw at the end of its next turn, becoming petrified on a failure or ending the effect on a success. The petrification lasts until the creature is freed by the greater restoration spell or other magic.

Unless surprised, a creature can avert its eyes to avoid the saving throw at the start of its turn. If the creature does so, it can’t see the medusa until the start of its next turn, when it can avert its eyes again. If the creature looks at the medusa in the meantime, it must immediately make the save.

If the medusa sees itself reflected on a polished surface within 30 feet of it and in an area of bright light, the medusa is, due to its curse, affected by its own gaze.

Actions

Multiattack. The medusa makes either three melee attacks—one with its snake hair and two with its shortsword—or two ranged attacks with its longbow.

Snake Hair. Melee Weapon Attack: +5 to hit, reach 5 ft., one creature. Hit: 4 (1d4 + 2) piercing damage plus 14 (4d6) poison damage.

Shortsword. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 5 (1d6 + 2) piercing damage.

Longbow. Ranged Weapon Attack: +5 to hit, range 150/600 ft., one target. Hit: 6 (1d8 + 2) piercing damage plus 7 (2d6) poison damage.

Legendary Actions

The medusa can take 3 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature’s turn. The medusa regains spent legendary actions at the start of its turn.

Attack. The medusa attacks with its shortsword.

Gaze. The medusa focuses its gaze upon one creature that can see it. That creature immediately makes a Constitution saving throw as if affected by Petrifying Gaze.

Snake Hair (Costs 2 Actions). The medusa makes a snake hair attack.

One More: Thug Boss

Thug Boss

Medium humanoid (any race), any non-good alignment
Armor Class: 11 (leather armor)
Hit Points: 48 
Speed: 30 ft.

STR	DEX	CON	INT	WIS	CHA
15 (+2)	11 (+0)	14 (+2)	10 (+0)	10 (+0)	11 (+0)

Skills: Intimidation +2
Senses: passive Perception 10
Languages: any one language (usually Common)
Challenge Rating: 1 

Pack Tactics. The thug has advantage on an attack roll against a creature if at least one of the thug’s allies is within 5 feet of the creature and the ally isn’t incapacitated.

Legendary Resistance (3/Day). If the thug fails a saving throw, it can choose to succeed instead.

Actions

Multiattack. The thug makes two melee attacks.

Mace. Melee Weapon Attack: +4 to hit, reach 5 ft., one creature. Hit: 5 (1d6 + 2) bludgeoning damage.

Heavy Crossbow. Ranged Weapon Attack: +2 to hit, range 100/400 ft., one target. Hit: 5 (1d10) piercing damage.

Legendary Actions

The thug can take 2 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature’s turn. The thug regains spent legendary actions at the start of its turn.

Attack. The thug makes a melee attack.

Taunt. The thug’s scathing remarks put one of its enemies off balance. It gains advantage on attacks against that enemy until the end of its next turn.