As my first guide(and hopefully not my last), I made a guide for a champion build for Morgana that I just used recently. Nowadays, I rarely see players use Morgana even as AP. One day I just saw her and we needed a support so I tried using her since it's not the first time someone did that and it's totally fun! Here's my overall build and view of Support Morgana that can still be lethal for late game.

Let's start off with Runes. Here is my standard set for my every support champions.

• Greater Seal of Scaling Armor - We all know that Morgana is a little too soft and so the additional defense is a big help for her alongside of her Black Shield
• Greater Glyph of Scaling Magic Resistance - Goes hand-in-hand with Seal of Scaling Armor. Again, for additional protection. Almost every team has their AP
• Greater Mark of Magic Penetration - Since you are a support, you don't need much damage so this is the only Rune for additional power. MR reduces your damage by a percentage, thus reducing it means you're increasing your damage by a percentage
• Greater Quintessence of Movement Speed - Morgana is a champion with low mobility. Nothing much too explain :)

Usually, supports go bottom lane with an ADC and because of that, CSing is almost not allowed. This is of course for the ADC to get it's core items fast. This is where Ancient Coin comes in handy. Upgraded, it can also be a support item because it has a little boost to you and your team's MS when activated. This is very useful for chasing running enemies and escaping if you are in a bit of a pickle. I also max out Morgana's CDR because she doesn't have a skill that is sure to land. You'll never know when you'll need that Dark Binding or Black Shield again so this is just perfect with 10%.

It is a standard for a support to have a Sightstone to have vision and avoid ganks/counter ganks. It also gives you a small boost in HP especially once upgraded. Boots are also important in an early game to get a better position faster to make your Spell Binding hit. I use Ionian Boots again for the 15% CDR(you now have 25% if you add the CDR from Talisman of Ascension).

Since you don't need much of a damage for being support, I tend to use Mejai's Soulstealer with Morgana. It's crucial to have this early on to max out the stacks faster. It gives you a decent amount of AP once maxed. If you gather enough gold, you can change this to Rabadon's Deathcap late in the game but I really prefer Mejai's because it's much affordable.

For late game items, I use Rod of Ages because of its HP/MP boosts. Good for additional toughness and you can also spam more skills. Another key item is the Will of the Ancients. It's got good amount of AP + CDR (all-in-all you now have 35%). The best thing about this is the 20% Spell Vamp well suited for Morgana who already have a 20% on her own. This will give you a 43% Spell Vamp if you'll be including Masteries. You can substitute this or the Rod of Ages with Zhonya's Hourglass to avoid burst damages and for additional AP.

Since you are a support, it is wise to max you Black Shield first to protect your teammates and yourself from those nasty situations. You can then max out your Dark Binding to deal more damage and to hold enemies for a longer duration. It is also important that you know how to place your Tormented Soil well for maximum effectiveness. I usually use Tormented Soil first before I use my Dark Binding. Once their in your Tormented Soil, it is only common for them to escape to avoid taking damage. This way you'll know the direction they'll be heading to land a sure hit with that first skill. You should react fast to still have them inside your Tormented Soil. This combo also goes for a full AP Morgana. You should also earn to distance yourself far enough to avoid your Soul Shackles from being cancelled. If you plan on targeting more enemies, it's best to hit one first with Dark Binding (I usually target melees or the softies), place your Tormented Soil in a nice way, Shield yourself and use your Ulti.

If you'll check the Masteries above, you'll see that my build is more on the Utility side. This is because you need to max out the effectiveness of your items such us trinkets and potions(more time means more heal since you don't have enough money to buy potions all the time). Mana regen is also crucial for Morgana since you'll be using it a lot if the enemy knows how to dodge your skills. Additional income is also important in the early game for you to have the needed items faster. Morgana is a low mobility champion so MS is very important. Again with the CDR, I max out Sorcery in Offense to have a full 40% CDR(can't go higher than that) along with the items mentioned above. You can change this dependiing on your preference.

For Spells, I use Ignite because ADC gets a Heal/Flash combo most of the time. This is very useful for dying enemies that is running to their turrets and for additional damage, too. You can also get Heal if the ADC didn't pick one. Barrier is also a good choice for that double shield. Exhaust is also a good choice to slow enemies down before hitting them with Dark Binding, or for the ADC to get a good distance to poke them, or to slow down enemies chasing a dying teammate. I always use Flash to escape through walls and brushes. All depending on your preference.