Hello there! I am a platinum player from eune, I used to main Morgana so I think I can tell you pretty much about her. Here is my Morgana guide, I decided to make this guide in order to encourage more players to play Morgana. If you don't know much about her, she is a champion of high CC and utility, she also has moderate damage and is very safe to pick if you are afraid of being countered, anyway let's get started shall we?

Flash is the one spell that you will see in every single game, just because it's the best summoner spell in the game. With Morgana you should use Flash for escaping and positioning yourself for a perfect Soul Shackles, or just to get in range of Dark Binding for a kill. The reason why I pick Teleport is that mid laners tend to roam a lot and since Morgana is a slow champion she might not be able to get to the lanes when the enemy is already there, this is why you can teleport for the gank or counter gank and save the day.
Getting Ignite on Morgana is viable, but since Morgana lacks mobility Ignite is not so good on her, Teleport will most likely aid your team more than Ignite Keep in mind that if they have Swain or some other strong healing champion, Ignite gives more than Teleport.
Below you will see a list of summoner spells that you shouldn't take in regular circumstances.
Cleanse is not very good on Morgana since you can use your Black Shield for CC immunity.
Barrier can be somewhat viable as a spell, but Morgana has enough toughness, so Barrier isn't the way to go.
Smite is really viable... for junglers, you are not a jungler so don't pick Smite.
you don't have any global skills so this will be pretty much waste of spell slots.
Clarity is more suited for low level players who aren't good at managing their mana, it shouldn't be picked on Morgana since her spells aren't very mana hungry.
Never.
pick this only if you are playing support, Exhaust is not suitable for mid.
Ghost is not bad for Morgana it beats Flash with duration and cooldown, but Flash gives instant positioning, when Ghost only makes it faster. I recommend Flash but Ghost is also viable.
Heal is not viable after it's nerf, and your passive Soul Siphon gives enough survivability.

I am not very familiar with the new masteries but what I understood from them, you should pick Expose Weakness to Morgana every time. Morgana has insane utility, but at the cost of not being able to carry the game alone (or atleast, very easily). This is why Expose Weakness is very strong on Morgana since it increases the damage your team does, and due to your AoE abilities you can proc it on many opponents.

The runes I use on Morgana are very good, but there are other very viable options.
But the reason why I chose these runes are because the Greater Seal of Armor and Greater Glyph of Magic Resist help you sustain damage from enemies, especially since you are the AP bruiser of your team. the Greater Mark of Ability Power and Greater Quintessence of Ability Power help you in farming with Tormented Shadow.
You can change the glyphs/seals for mana regeneration which gives you almost infinite lane sustain with Doran's Ring and Soul Siphon

spaaaaaaace

Pros + Strong utility for the entire team + a well placed Soul Shackles wins teamfights with ease + Tormented Shadow makes farming too easy + Long lasting bind and AoE stun with Dark Binding and Soul Shackles + Black Shield gives CC immunity to anyone on your team. + Nearly impossible to gank

space

space

Cons - Morgana has cons? - Morgana has low mobility - Low late game damage - CC is not realiable Dark Binding is a skill shot and Soul Shackles can be escaped. - Tormented Shadow needs good placement to deal any damage in teamfights

Morgana has a very simple laning phase. Morgana will win probably any mid laner in a farming competition, right after you get level 5 and your Tormented Shadow to level 3 you can kill the caster minions with a single cast. Harassing the opponent requires that you land Dark Binding and following up with Tormented Shadow. This is somewhat useless since Morgana is all about farming and using your utility in the late game to win games, snowballing is a possibility but Morgana's damage falls in late game so carrying will be hard with all the kills on you, this is why you should try to give kills to allies when you gank and focus on farming when on mid. After you reach level 6 you might want to roam top or bot for ganks. Initiate with Soul Shackles because of the slow, making it easier to land your Dark Binding.

I like to rushing Rod of Ages because Morgana is at her best when she is tanky, and since Zhonya's Hourglass is not required at this point of the game, but should be rushed afterwards. And because we are playing tanky Morgana we want to build Liandry's Torment and Abyssal Mask for their defensive stats connected to utility and damage.
Why should you get Mercury's Treads on Morgana? Sure you have Black Shield to counter CC but it won't last forever and in teamfights some carry champions might need it more than you, and this is why you need tenacity to prevent from being CC'd in place, which possibly ruins your ultimate if you lack tenacity.
Rabadon's Deathcap might sound weird but even though Morgana loses damage in late game, it doesn't mean that you should delibirately take away full AP items from your build. Rabadon's Deathcap gives you a nice boost in damage, but it can be replaced if the situation calls for something like that.

Teamfight is the place where Morgana does her best, your ultimate is AoE slow/stun and some nice DPS to the enemy team, you shouldn't rush the ultimate but neither should you save it to the last moment. If you have Flash available you can make an awesome initiate by flashing into their team with you Black Shield on and using ultimate in order to attach as many chains as possible. When you pop your Soul Shackles and they focus you right away instead of running, activate your Zhonya's Hourglass if they are running, then follow them for your Soul Shackles stun. Morgana has long cooldowns in her abilities so try to position yourself for a bind when you don't have skills to use.

Soul Shackles can only be used when enemies are nearby. Your HUD will show that it's available when near an enemy. If an enemy is in stealth, your HUD will still show that your ultimate is available, what this means is that you can "reveal" stealthed units with your Soul Shackles
With the change in Morgana's Tormented Shadow you can farm the caster minions with a single Tormented Shadow already when it's at level 2
Tormented Shadow deals damage instantly when you cast it so you can use it to pick up a low hp enemy.

That's it, my Morgana guide. I hope that it will encourage you to pick up Morgana although she is underplayed and rarely seen, just keep in mind that she is a very potent champion and should not be underestimated.
If you have any questions or improvement suggestions for this guide feel free to post a comment and tell me about it.