It's a spell of a unit of ours, Skink Starseer. I would strongly recommend you to buy our book or search through the internet, gather all of our units profile and study them in order to be able to make your lists ! Judging by the other post you made, I believe that purchasing the Generals' Handbook 2017 would also be really useful to study in order to be able to fully understand what our army is capable of !
Basically it gives you a +1 or -1 on a single dice roll each phase (also enemy phase) for the unit that you cast it on. I recommend searching for the Starseer warscroll.
To further elaborate on that: It doesn't seem too powerful on first glance, but it is not bad at all. One of the best examples is charges: the unit cannot fail a 3" charge anymore. Or if you look at the Engine of the Gods: with a Slann near and using that spell you have much more control over the Engine's ability, making it much more reliable. Combined with a Balewind Vortex and an Astrolith Bearer it makes Kroak's lowest possible casting roll a five (!!) Which essentially means you cannot fail arcane bolt, even without rerolling. When cast on a Slann it makes it more likely to roll that double for the extended spell range. When carrying the Incandescent Retrices Item the chance on that bad result is halved. When a Carnosaur hits with its jaws, buffed by a Serpent Staff, you could increment a dice showing a five to make it a six, to do double damage. Those are just a few examples that show how powerful adding just one point can be.
Fun fact: a Skink Chief had that buff permanent active for himself. Pretty cool. ....sad he is gone, even without his command ability he was kinda cool. Edit: Fun fact: my memory is significantly worse than I thought. He didn't have +-1 He had rerolls...
I like all your examples but given them I would say it's not "Not Bad". I think it is possibly top 10 best spells in the game. It's crazy easy to get off, it has a range outside dispel range and it targets a "unit" with no specifications. If you are playing on a team, it is so unbelievably powerful it crazy. I played with beastclaw the other day. Loremaster mixed with starseer (loremaster also having one of the best spells in the game)? Sweet mercy. You can take a stonehorn that is very strong and turn him intro an absolute nightmare. We dealt almost 50 wounds on the charge (I mean...I know, loremaster gets most of the credit). Also, it allows you to deal 4 damage on a d3 wound roll. I find that funny. I think I just like the ability to fix 'one mistake'. I love the fluff and the usefulness. And it modifies weird things that nothing else modifies. I've been using it to make engine of the gods better. It can fix a "Roll to see if something gets off" ability that nothing else can modify (looking at you thundertusk). It fixes prayers that go south or the really unfortunate roll-1 reroll-1 that we've all experienced. Want more Plague Drones? Battle Shock of 2? Starseer to the rescue! It's my favorite spell, so I'm probably giving it too much credit!
It's incredibly useful also if you cast it on an enemy's unit, when you're targetting it. It's going to fail a save in the shooting phase, in the combat phase will fail an attack or a save, it will get nastier results in the battleshock and so on.