1). You missed the Rune Sword and Rune Shield. The gimmick of both is that they increase your Magic Defense, but are otherwise not particularly noteworthy (the shield, in fact, has the lowest durability in the game). If you still want them, however, just leave the level and come back, and make your way back to where Ostrava appeared; you should see the items there. Both items can also be found in the second part of the Tower of Latria.
2). No, he's just a Black Phantom. You should be safe.
3). Don't feel too bad if he's giving you trouble, he's one of the more infamously difficult bosses (I've never understood why, though :shrug:). Here's a tip, equip yourself light enough to roll. If you see Allant rear back and look as if he's about to grab you, stop what you're doing and roll the [censored] away, he's about to use the Soul svcker which saps you of you Soul Levels. Generally put, keep a distance away from him and wait for him to
approach charge you, and roll out of the way of his slashes. Get a few attacks in while he does this, and then widen the gap. And if you see him stand still, stab his sword into the floor and start glowing GET AS PHYSICALLY FAR AWAY FROM HIM AS YOU CAN! He's going to summon a blast that will engulf much of the battlefield, and it's unlikely you'll be able to survive.
4). World Tendency doesn't exactly change based on your morality. Worlds will darken if you:
-Die in body form.
-Kill a named NPC (like a vendor)
-Invade another game as a Black Phantom and kill another player.
And they'll brighten if you:
-Kill a boss
-Kill a Primeval Demon or named Black Phantom (both of which usually only appear at pure black tendency)
-Kill any Black Phantom invaders
-Summon Blue Phantom players to assist you
-Be summoned as a Blue Phantom in another player's game and successfully kill the boss.
Additionally, while online, the tendencies will be base themselves partially off of the average of all other games (so they'll usually shift toward gray). And sometimes, during holidays, special server events will occur that set the tendencies either towards Pure White (such as during Christmas) or Pure Black (during Halloween).
5). Yes, in the same way as all other enemies. Darker world tendencies make them hit harder but yield more souls when killed, whiter ones make them easier but yield fewer souls.