(That's a DM call, I'm afraid. AoO only requires a melee attack. It doesn't stipulate weapon or spell, and war caster allows you to cast any spell so long as it requires only an action and targets one creature.)
(OOC: Okay, Sage Advice says no. Opportunity attack doesn't allow that attack to be a melee spell unless you have War Caster. So I guess Magnum doesn't have a weapon ready? Can he do an unarmed strike? If so, do you want him to? We can use the same roll as above.)
The Hyena dies in a flash of lightning as impressive as the last few were pitiful.
Combat is over. A dead guard lies on the ground, as does the guard that lies unconscious, stabilised by Silent. The male gnome merchant cradles his wife; she is whimpering softly, clearly in great pain. The horse, dead before you arrived, also lies on the ground, pulling the wagon down towards the ground slightly. There is a shocked silence for a moment.
The young human guard jerks awake as if you had struck him. He sits upright. You see his terrible wound has healed, not perfectly, but he will live. He looks up, confused, at the two gnomes, and then he looks around. I... Where's Morum?
Silent stabs his blade into the dirt to leave it standing and wordlessly walks over to the body of the guard. He checks his pulse, realizes that there's no saving the man and turns to the younger guard.
I'm sorry - he's gone.
He'll unhook one of the handaxes from his belt and attempt to use it (along with his hands) to start digging a hole.
The guard walks around the cart and sees the body on the ground. He watches as you check for life signs. Then he walks to the body, drops to his knees, and puts his hand on the other guard's chest. I... it takes time before he can speak. His family moved to our neighbourhood when we were both five. We've been best friends ever since. We always talked about going out to see the world and...
He bends over the body and just begins to cry.
Silent, the gnome merchant waves you over to the rear of the cart; he is tending his the woman's foot, and you would guess that they are married.
There's no need for... he points to the beginnings of the hole you were digging... that. We'll turn back, take him home to Nicodranas. He also has tears in his eyes, from shock, or grief at the death.
Silent nods and sheathes the handaxe. Afraid we don't have any healing potions to spare, and I ain't never been much good at field medicine. Miss, you think you'll be alright for the rest of the journey?
I'll be fine, the woman says. Thank you, dear, she says to her husband as he puts the finishing touches to the bandage he has applied to her wound.
We'll need to clean and dress it every day until we get home, the man says, but... I think it will be okay. He looks up and over at the two guards. Poor Morum. And poor Crane. Closer than most brothers I know.
We'll get the lad to his family, the woman replies. I'm Greta, and this is Gerter my husband of fifty years and counting. You and... your friend saved our lives, you know.
(deleted)
(OOC: Did Magnum take his opportunity attack on Hyena 3?)
Silent runs up behind Hyena 3 and takes a strike at its legs.
Attack 25 Damage: 13
(Deleted)
(deleted)
Magnum's Magic Missle kills Hyena 2. Magnum, are you taking an Opportunity Attack on Hyena 3 as it moved away from you?
Magnum grabs at the Hyena as it tries to leave his reach, hitting it with a [spell]Shocking Grasp[spell].
Attack: 20 Damage: 4
(OOC: Does Magnum have a feature that allows him to cast a spell as an Opportunity Attack? Normally that requires the Warcaster feat.)
(That's a DM call, I'm afraid. AoO only requires a melee attack. It doesn't stipulate weapon or spell, and war caster allows you to cast any spell so long as it requires only an action and targets one creature.)
(OOC: Okay, Sage Advice says no. Opportunity attack doesn't allow that attack to be a melee spell unless you have War Caster. So I guess Magnum doesn't have a weapon ready? Can he do an unarmed strike? If so, do you want him to? We can use the same roll as above.)
(OOC: Magnum has no melee weapons active, so he'll try to take an unarmed swipe at the Hyena. If it hits, he only does 1 damage.)
Attack: 15 Damage: 0
He does 1 damage. :)

And then Silent finishes off Hyena 3.
Only Hyena 1 remains. It kills Silent's Echo.
The merchants are two distressed to do anything, even with Hyena 3 dead.
Guard 1 fails a saving throw with a crit, and dies.
Magnum is up. Hyena 1 is very close to death.
Electricity crackles in Magnum's free hand as he fires his Lightning Launcher at Hyena 1.
Attack: 14 Damage: 8
Additional Damage from Lightning Launcher: 3
The Hyena dies in a flash of lightning as impressive as the last few were pitiful.
Combat is over. A dead guard lies on the ground, as does the guard that lies unconscious, stabilised by Silent. The male gnome merchant cradles his wife; she is whimpering softly, clearly in great pain. The horse, dead before you arrived, also lies on the ground, pulling the wagon down towards the ground slightly. There is a shocked silence for a moment.
What would you like to do?
I'll use my last spell slot to cast Cure Wounds on the unconscious guard for 7 HP.
The young human guard jerks awake as if you had struck him. He sits upright. You see his terrible wound has healed, not perfectly, but he will live. He looks up, confused, at the two gnomes, and then he looks around. I... Where's Morum?
Silent stabs his blade into the dirt to leave it standing and wordlessly walks over to the body of the guard. He checks his pulse, realizes that there's no saving the man and turns to the younger guard.
I'm sorry - he's gone.
He'll unhook one of the handaxes from his belt and attempt to use it (along with his hands) to start digging a hole.
The guard walks around the cart and sees the body on the ground. He watches as you check for life signs. Then he walks to the body, drops to his knees, and puts his hand on the other guard's chest. I... it takes time before he can speak. His family moved to our neighbourhood when we were both five. We've been best friends ever since. We always talked about going out to see the world and...
He bends over the body and just begins to cry.
Silent, the gnome merchant waves you over to the rear of the cart; he is tending his the woman's foot, and you would guess that they are married.
There's no need for... he points to the beginnings of the hole you were digging... that. We'll turn back, take him home to Nicodranas. He also has tears in his eyes, from shock, or grief at the death.
Silent nods and sheathes the handaxe. Afraid we don't have any healing potions to spare, and I ain't never been much good at field medicine. Miss, you think you'll be alright for the rest of the journey?
I'll be fine, the woman says. Thank you, dear, she says to her husband as he puts the finishing touches to the bandage he has applied to her wound.
We'll need to clean and dress it every day until we get home, the man says, but... I think it will be okay. He looks up and over at the two guards. Poor Morum. And poor Crane. Closer than most brothers I know.
We'll get the lad to his family, the woman replies. I'm Greta, and this is Gerter my husband of fifty years and counting. You and... your friend saved our lives, you know.
We are also on our way to Nicodranas. Perhaps we should assist them in their journey. What are you transporting?