Ways To Avoid Backstabs
On the islands of sloe and Kordan backstabs can be a touch annoying, but they won't kill you in most cases. This all changes with the third island of Archais. The first really nasty backstab most people run across is from the Brown Scorpion in the Shadowclaw swamp. This gives you a taste of what to expect on the mainland, and it is ungood. There are some things you can do to help either avoid the backstab or take less damage, and I'll go over them in this post.
The Cleric Spell Sense Life
This is the one spell in game which specifically helps you avoid backstabs. It is quite easy to unlock as well only requiring level 9 cleric with 17 wisdom and 13 charisma. It only cost 15 mana to cast making it quite cheap, and it is maintainable if you know the mage spell maintain.
Overall it is a good spell to have ready if you are going somewhere with backstab potentials. It isn't a 100% avoidance, but nothing ever is. The key is to layer up different things to help protect you and reduce the chances of eating a big one. Sense Life is just one of the options you have available to ensure that doesn't happen.
The Thief Skill Dodge, and Dexterity
No matter where thief is in your class setup, it is very important to have the skill dodge. It is unlocked at level 2 thief, so everyone should really have this one. It is passive, and means you'll generally take less damage in combat, and this includes backstabs. If you can also pare this with a decently high dexterity (20 or above) you can expect to get a good boost to your backstab avoidance. The higher your dexterity is the better when it comes to dodge. Personally I like to bump my dex up to 25 as soon as possible, but 20 should be fine for most people.
Like with sense life this isn't going to make you avoid all backstabs, probably not even close, but it helps and that's really the best you can hope for. It is just another couple layers you can add into your protection. The next options is actually what I've found to have the highest success rate, but it takes a little bit more time and resources to get up to where it works.
The Thief Skill Sneak, and The Semicolon
Note: You'll want to have the mage spell infravision for looking into neighboring rooms no matter the lighting conditions. This ensures you can always see what mobs are waiting for you in a room before blundering into it.
Sneak is a bit different because it doesn't help you passively avoid backstabs, but it allows you to make the first attack. If you can hit a backstabbing mob first, even if you miss the attack, they are unable to backstab you. For this to work you first need to learn the thief skill sneak which unlocks at level 5.Then you have to spend some combat points to train it up, or get it to go up naturally by walking back and forth near a strong mob and hoping it goes up. I've found it is much easier to just spend the combat points. Each percentage only costs 10 points, and if you can get it to 93% or 94% you should be able to sneak just fine, as long as you aren't carrying or wearing anything that hums. You can check your sneak level by entering in 'show sneak', and you can toggle on and off your sneaking status with the command 'sneak'.
The second part is to move into a room and attack as quick as possible, and this is done by putting two commands on one line. You do this with a semicolon separating your two commands. For example the command 'Northeast;cleave scorpion'. This would walk you north east and hit the scorpion in that room with a cleave the second you walk in. This is much faster than sending two different commands to the mud. If you have a decent amount of sneak knowledge you will almost always succeed. This only works if you know the mob in the next room is a backstabber though, and doing this with every mob on every map is a bit too much. So though it has it's uses, it isn't for every situation, and you'll still have to rely on your other passive backstab defenses.
Skeletal Spiders
These are useful little minions, and because the level requirement is so low they can be used by pretty much anyone no matter their class setup. They are unlocked at level 3 necro, and they have a pretty low intelligence and wisdom requirement as well. The kicker here is you'll need to know the thief skill sneak as well, or they will be nearly useless. In addition to sneak harvest teeth will be pretty necessary so you have teeth to use to make the spiders. Harvest teeth is also a low level requirement though, you can get that one at level 2 necromancer.
The way this works to help you avoid backstabs is through using spiders as decoys. If you put out as many spiders as you can, then make sure you are sneaking, you'll be last picked for the mob to backstab. This is because you are being sneaky, and the spiders cannot sneak, so they get picked and they eat the backstab for you. They will die after one hit, but it's okay because they have done their job, they are just decoys. I still use this on certain maps if I don't want to bother fighting with a series of backstabbing mobs.
Sanctuary & Normal Saves
There are actually two articles dedicated to this topic, and I'll link them below. All you need to know is sanctuary is your best friend when it comes to backstabs. Chances are you are still going to eat some, and a sanc will reduce the damage by a hell of a lot. Having some normal save will also decrease the damage, and enough will make it tickle instead of making you dead. Granted this is only the case if the backstab is coming from a normal weapon, if it is a nonorm stabber then use everything else listed above to avoid it and hope for the best. Article on defensive spells. Article on normal saves.
Conclusion
Like with most defensive options in alter nothing is going to do a lot on its own. The key is to make multiple layers of protection. This way if one fails you have another to fall back on. Commonly all will fail and you'll still get whacked with a backstab. In these cases you rely on your sanctuary and normal saves to reduce the amount of damage you actually take. If you can do all the above you are almost assured to not get one hit killed from that wonderful brown scorpion, or from other backstabbers on the mainland.