Mimics are amazing in any adventure! But there are ways to make the even cooler.
1. Make them speak - If you read through mimic lore, they can speak common and other languages including their own special mimic language. A mimic that speaks to the adventurers is pretty cool. This could even lead to the characters trying to learn the mimic language. And if they have a language, does that mean there is an entire mimic society? That would be cool.
2. Make them smart - Though in 5th Edition they only have a 5 Intelligence score, this wasn't always there case. In 3e they had a 10 Intelligence and a 12 Wisdom. This means they are as smart as most adventurers and even more wise. Make them crafty and sneaky. Give them some skills. When they fight in battle, they use tactics and strategy. They would rather surrender or negotiatie than fight to the death.
3. Put them outside - They were originally supposed to be subterranean and hate sunlight, but not in recent editions. A log crossing a stream makes a perfect mimic. Halfway across and they attack... boom! Maybe they are a fire ring or rock at a campsite. Maybe they are a road sign that points in the wrong direction into a trap. Maybe they are a lamp post in town or a bathtub.
4. Make them friendly - Mimics could be quite friendly in early editions if you offered them food. There is a good chance that the mimic might know a lot about the dungeon or its residents. Maybe they are willing to travel along with the party, or maybe they are lonely. Adding a mimic to the party as an NPC could be a lot of fun and give the characters a lot of options to solving encounters.
5. Use them in groups - One mimic makes itself an obviously fake chest and the other poses as a table with a key sitting on it. The characters will focus their attention on the chest and the table can attack from behind! They are smart, so they can definitely work together. Maybe there is an entire colony of mimics that lure unsuspecting adventurers in and then surround them.
6. Make them experts on treasure - They were originally created by wizards to guard treasure which means they have a lot of experience with it. Make your mimics experts on treasures and magic items. Maybe they know all about the Staff of the Magi or the value of a set of diamonds. Maybe they can even identify magic items if the characters would be simply willing to set the magic item inside the chest that the mimic becomes.
7. Tag team wrestlers - They are great grapplers. When they are used in groups (like I mentioned above) they can be powerful. Grappling and attacking together as a team is a very effective way to defeat the characters. They could also make great allies with these skills. Imagine a mimic grappling the bad guys for the characters to attack. Or maybe they hold off a few minions while the party attacks the BBEG.
8. Create baby mimics - Little tiny mimics are cute and adorable. Teenage mimics can also be really fun. They don't all have to be adults. Young mimics can be silly or imaginative or adventurous. They don't just sit around pretending to be chests all day long.
9. Give them class levels - Remember, they can be smart!. Imagine a rogue mimic with some stealth skills and the ability to bonus action dash. What about a barbarian mimic making a reckless attack while raging? What about a wizard mimic who learned spells from its creator? Could it be there is a mimic deity that mimic clerics serve?
10. Team up with other monsters - Any acid based monster makes a great partner for a mimic. What about a mimic chest with a green slime inside it that it can spit at the characters? How about a mimic sarcophagus with a zombie or skeleton inside? A well mimic containing a water weird... A black dragon wyrmling raised by a mimic...
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