Is this the appropriate spell level?

[NerdHut]

There's a spell from a 1990s video game I love which I'm suprised doesn't have a good analog in D&D 3.5, so I'm homebrewing one up. But since I've never made a homebrew spell before, I'd like some guidance. I have the effects I want, but I'm not sure if it's too powerful or weak for the spell level I chose.

Magic Bomb
Evocation [Force]

Level: Sor/Wiz 3
Components: V, S, M
Casting Time: 1 standard action
Range: Close
Target: One 5-ft square
Duration: 1 round/level or until discharged (D)
Saving Throw: Reflex half
Spell Resistance: Yes

A magic bomb spell occupies a 5-foot square of ground or floor. The bomb detonates when a creature enters the square (or when the spell is cast, if it is placed underneath a creature), dealing 6d6 points of force damage. Any creature in an adjacent square to the bomb takes half as much damage.

You and any characters you specifically choose can pass over the bomb without activating it (though they may still take damage from the area effect). To any other creature the magic bomb is nearly invisible, but you and the chosen characters can see the magic bomb’s location as a faint flame which casts no light.

You can remove the bomb whenever desired. Another creature can remove it with a successful dispel magic, but attempting to dispel the bomb and failing to do so triggers the explosion.

Note: Magic traps such as a magic bomb are hard to detect and disable. A character with the Trapfinding ability can use the Search skill to find the bomb and Disable Device to thwart it. The DC in each case is 25 + spell level, or 28 for magic bomb.

Material component
A short string treated with sugar of lead

I used explosive runes as my starting point and tweaked it till it felt right.
If it seems too powerful, I'd rather nerf it than bump up the spell level. But any input (including spotting spelling/grammar issues) on how to improve the spell would be appreciated.

[DeTess]

I feel like this needs some buffing (duration to minutes or more per level, I think), or a damage reduction and downgrade to level 2. As it stands I'm having a hard time figuring out why I'd want to cast this over fireball.

[Vaern]

I'd buff the duration a lot. Maybe just go straight for permanent until discharged. 1 round/level just doesn't make for a good trap spell.
Honestly, since it's effectively an explosive runes variant that pops with proximity rather than interaction, I'd just make it deal the same damage (full with no save in the middle of the blast, reflex half out to 10 feet). This spell could arguably be a bit more powerful simply due to targeting a space rather than an object and discharging more easily, but I don't think it's so much more powerful that it would warrant increasing the spell level.

[DeTess]

I'd buff the duration a lot. Maybe just go straight for permanent until discharged. 1 round/level just doesn't make for a good trap spell.
Honestly, since it's effectively an explosive runes variant that pops with proximity rather than interaction, I'd just make it deal the same damage (full with no save in the middle of the blast, reflex half out to 10 feet). This spell could arguably be a bit more powerful simply due to targeting a space rather than an object and discharging more easily, but I don't think it's so much more powerful that it would warrant increasing the spell level.

If you go this far regarding the duration I would place some sort of limit on the amount of magic bombs you can fit in a small space (for example, you can't place them within 10 feet of another). This is to prevent 'stack 100 of them together for all the damage' abuse.

[NerdHut]

I can definitely see extending it to minutes per level, or even 10 minutes per level, but nothing longer than that. The reason I defaulted to rounds per level was that in the old game it was primarily a combat spell, though it could be made into a short-lived trap. With that in mind, I wanted something that served as a trap second, not first.

The spell from the video game affects one creature at a time, two if you're very lucky. Too big of an area and it just feels like I'm remaking fireball. It's the same reason I didn't switch it to 1d6/level damage, though some amount of scaling damage may be a suitable buff.

I'm glad you mentioned the limitation on placement, DeTess, because I hadn't thought of that. As is, I suppose it could still get abused via stacking, and even worse if I extend the duration.

[heavyfuel]

Force damage is nice, but it's not nice enough to justify this spell's level and - most importantly - lack of scaling.

6d6 Force damage is decent when you get it at level 5 (presumably), but it very quickly becomes useless.

If you don't want to remake Fireball, give it a scaling similar to Barkskin's. Say the spell deals 3d8 damage and additonal 1d8 for every 2 caster levels above 5, to a maximum of 10d8 at level 19.

It scales slowly enough that it won't outshine other spells, but steadly enough that it will never be a useless spell.