War Frogs! Okay. I really like these guys. They are nasty. Imagine living in a swampland. Horses and dogs are not effective in the soft and watery environment. Imagine a species of frog that is intelligent, can blend into the environment making it nearly invisible, navigate difficult terrain as easily as using a road and the ability to attack unwary victims with an acid attack. Yeah, the war frogs are calling.
I introduce a faction of Northmen that lived in the swampy areas in the Komor Forest called the Myr Folk. They bred these frogs to act as guards, much like city folk use dogs. But better.
The party comes across an old Northman fort, damaged, but still intact. The acid from the war frogs has caused most of the damage. But this isn't just a fort, it was once a temple to Arom, slayer of the great poison god. Within the fort is what remains of his spear.
Approaching the fort should set the tone. Low level fog creeping around the players ankles, the dark walls of the fort looming over head, the air full of the buzzing of large insects and a cacophony of frog song. Each step in the swamp should stir a creature to move or hiss out a warning. I'd add in a few large snakes that slip in and out of view using the swamps vegetation as concealment. A large ripple disturbs the black water's surface distorting the reflection of the moon.
Use the war frogs full tactical ability. Use their camouflage to get surprise attacks, use their ability to climb sheer walls to keep out of melee range, have them spit their acid, and don't be afraid of the frogs focusing on one character at a time. These frogs were bred to fight and kill. They have had the instinct of fear removed. They attack until killed.
Capturing young war frogs, I'd allow that 1d3 would survive, and training them as a faithful pet adds a cool addition to a party. Hell, as GM, I'd even allow a halfling to use one as a steed. I think players would enjoy the oddity of it. A war frog is very useful, but not over powering. It has a decent amount of hit points, but one solid hit can take them out.
The artifact has no magical abilities, but it is a significant object in Northmen culture. However, the deadly poison is still potent and kills on contact. The players might figure out how to use this to their advantage. Of course they will. However, even using a gloved hand or some sort of barrier makes it still extremely dangerous to work with. I'd give a +2 to saves if someone with a protective bearer grabs the spear. A failure still result in death. But if they figure out a way to weaponize it you can minimize the power by saying that now the wood is exposed to the air once again it begins to decay quickly.
This adventure in a way is about balancing something cool. Frogs or poisoned artifacts. Both have very useful qualities, but still need to no overpower the game. So using this adventure a GM might want to come up with their own solutions before the players get their hands on them, but also not punish them for being inventive.
I've included the PDF a map for the GM and one for the players. I hope you can find a place for this in your campaign or maybe a fun encounter in a night of random gaming.
Thanks and enjoy!