In past editions of DnD, when a party faced off against a lone BBEG (Big Bad End Guy), the poor beastie was typically overwhelmed by the sheer number of actions or turns the party would take compared to the monster. Which tended to make for some anti-climatic final battles.
Happily things have improved over the years, and the DnD 5e Monster Manual provides a very respectable system to give solo type monsters a fighting chance by giving them “Legendary” abilities.
To summarise, Legendary monsters get a small number of points every round to spend on off turn actions in between PC turns (more powerful abilities cost more points to use). Legendary monsters also have access to a limited number of automatic saves (usually three), and “Lair” abilities and actions that generally occur on a set initiative count (independent of the monster itself).
In combination these are great ideas and go a long way to making a fight with a lone BBEG scary and memorable. Unfortunately the Monster Manual includes very few Legendary Creatures, and they all tend to be high level monsters of “legendary” renown.
So where's the problem? Well, what if you want to make a BBEG out of an Orc boss? Or a Cult Fanatic? Or some other low or mid level monster for a 3rd level party? The problem is if you're playing low or mid level, the Legendary mechanics aren't really available to your monsters. You can hardly throw a CR 16 Legendary Mummy Lord against them.
So what can you do? Thrown down House Rule #3: Solo Enemy Template of course!
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