My character was a level 6 wizard named Elonus, the rest of the party consisted of a monk, cleric, and barbarian of the same level. We had been doing odd jobs, fighting the big bad's minions, when we got a message from the Mayor a town we visit often. "Yindril has sent an army our way, we need as much help as you can muster. They are only a week away!" We dropped everything and headed straight there for 6 days straight. We passed towns and grabbed volunteers, friends, mercenaries, whatever we could get our hands on.
We ammassed roughly 40 people, some just commonfolk, some less powerful adventurers, some guards from towns we have helped, etc. The town was fortified to a comical extent. Trenches, barricades, towers, the works. We had only a day to prepare. We met with the city council and discussed a plan.
All we knew was that the army coming was sent by Yindril (the big bad), likely comprised of undead, led by Wrath (one of Yindril's demonic commanders), and was roughly equal in size to the town's military force: 3,000. We decided that since we have the walls and defenses we would be fine, so we just had to defend. How little we knew…
As we waited out the night, our dm gave us plenty of hints and chances to leave, or avoid the conflict. In hindsight, he probably didn't expect us to actually stand our ground, and knew this amount of combat would likely kill all of our players. But we didn't realize it until it was too late. We saw the forest trees shake and creak and a massive army of zombies began to come out in organized groups, being controlled by a massive fiery demon: Wrath.
We fought alongside the town trying to keep them back, but as more and more kept coming, we realized our estimates were wrong. There were at least 12,000 zombies out there. They overran the army outside in minutes, now only 1,000 of us were left inside the town as we desperatly tried to keep them outside the walls. Our dm seemed to be trying to get us to leave, he knew we could not survive it. But we were an inventive group, we decided to stay and figure out a way to either win the battle, or get everyone out of the town safely.
The dm prepared a solumn tone, saying "The clouds get darker as you hear thunder in the distance, the trees begins blowing side to side in the wind as it begins to rain". He kept talking, but something he said gave me an idea, thunder… trees… THATS IT! I had two 3rd level spell slots left and I knew exactly how to use them. It came to my turn, "I cast fly, and get as close to the forest as I can". I'm able to get there, and now it sinks in that what I am about to do might get me killed, I have 1 spell slot left… If I got attacked and lost concentration on flight, thats it. I patiently waited, hoping to not get attacked. Thankfully, I did not. The dm, now realizing I had some sort of plan, asked coutiously what I would like to do. I ask if I could align myself so that i have a straight line going through as many trees as possible, as close as possible to the center of the zombie army. "Roll investigation" he says. I roll, 17. "You are able to find an angle where there are 132 trees going in a line through the center of the army." he says. I smile, saying to the party and the dm, "You guys ever see lightning hit a tree?" the dm gives me that "no way" look as he realizes what I am about to do. "I cast lightning bolt straight through the trees". The dm realizes that trees don't have a dex stat, so they can't roll a save. Instead he has me roll against their ac, nat 20. After the table erupts in exitement the dm takes a second to determine what the damage would be for an exploding tree. He decides it does 8d6 peircing in a 20 foot radius (bassically fireball with pericing damage). I roll, 42 damage. Multiply that by 132 trees, double it since it was a crit, and we were left with 11,088 damage against the army. The zombies in the army were designed to be weak along, but strong as a group, so they only had 5 health each. He describes what happens: "As you all look down from the wall, you see Elonus floating there, as lighting leaves his hands you instantly see a flash of bright light and the sound of explosions and shrapnel. When you are able to get a clear sight of the battelfield, you see a large section of the forest blown so smitherines, and the bodies of over 2,000 zombies laying covered in wooden splinter." He looks to me "As you expend your last bit of energy, you see Wrath turn towards you quickly, almost with a look of disbelief".
As we sat around the table our monk mentioned "Wait, 2,000 zombies, shouldn't he level up?" the DM thought for a moment, and said "I suppose so, I'll say that you all level up from just watching that!" we all laugh and continue playing, and it gets to my turn again. Since I leveled up, I got all my spell slots back, so I begin flying back to the wall, thats when I remember. 9 sessions ago I copied down the banishment spell into my book, but I never had a 4th level spell slot to use it with. But now I have 1, and we all know just who to use it on. We don't quite have the range on Wrath from the wall to use it, so we devise a plan over the next few turns. Our barbarian leads the remaining soldiers to the gate, the cleric gathers all the healers he can find, and the monk stands near me in the center of the formation of soldiers. The barbarian pushes the gate open and the entire army begins spearheading towards Wrath through the army of zombies. Soldiers and our Barbarian on the outside are constantly healed and supported by our Cleric and the healers. As our army pushed through, we made sure to have the soldiers protect and secure a path back to the town for our escape.
We got within range, I cast Banishment. The dm rolls the save behind the screen and describes what happened. "As you bend a wave of arcane runes and lob it at this giant demon, you see material bend like a black hole around him, as he gets sucked back to his home plane." Its almost over, all I need to do is not get damaged. We follow the path back into town that the army kept clear, get inside, and shut the gate. The monk and rest of the party grouped up around me and kept them away as we retreated as far towards the center of town as we could. There were some close calls, but after a minute the party doing everything they could to stop zombies from hitting me and canceling my concentration, the dm speaks. "As you make your last stand against this dark army, zombies breaking through the walls and heading towards you from each direction, the drop, a magical essence leaving their bodies as Wrath's connection to this realm is officially severed. Cheers ring out throughout the town, for the first time in years, a beam of hope has fell upon the city." And we ended the session there. I was usually more of a roleplay guy than a combat guy up to that point, just because I treated it like just damage enemy, get damaged, wait for one of you to die. But that day was how I learned that combat can be just as interesting as roleplay if you think outside the box (and have an awesome DM that is willing to follow the rule of cool to allow such moments to happen).
TLDR; Shot lighting through a forest filled with a zombie army, blew up the trees, killed 2,000 zombies, learned to love combat.