Krobelus has seen her ups and downs throughout the years, considered to be from completely ignored to first pick/ban. While her three main skills never sounded very intimidating, her very feared ultimate was a weapon of destruction not to be underestimated. Taking down towers and heroes with extreme speed and ease, as long as she was kept alive, was an easy task. With mobility and crowd control items making their way into the game though, perhaps she’d become outdated; no matter the amounts of damage a hero can deal, it doesn’t matter if it can be avoided so easily.
Her lane control was pretty good and she could stand up to most mids. A spammable, relatively cheap AoE spell was enough to secure farm and rune control, and keeping her unchecked would result in a very ruined tower in the incorporeal claws of her ghost sisters. Even carries could feel very underwhelming if they tried to fight her head on, while a well placed Silence could make sure she wouldn’t be nuked down. Witchcraft was also there to buff all of her other spells, plus her movement speed.
With the removal of Witchcraft, she received a fair amount of buffs similar to what it used to provide, to make up for it. Her new spell, Spirit Siphon, sounds (and most likely is) quite the weapon. The percentage of HP stolen grants extra survivability, which she badly needs, while also making for an additional damaging tool that scales well into late game.
The hero was falling out of shape, I won’t lie. The large amount of survivability/mobility potential that was kept being added in the game made it pretty hard for her to use her full potential on an actual hero. Taking towers easily is always good, but it tends to get hard with 5 people nuking the life out of you. And while she was pretty darn fast herself, that didn’t really help much with her spirits catching up to people. Decent opponents would kite her effectively, take advantage of the long ulti cooldown and make their move.
Now, let’s see exactly what she was given. At first glance, Spirit Siphon may sound utterly broken. Then again, most new material does at first, so let’s bear with it for a change. This one greatly boosts her laning stage, which has proven to be very deciding in the recent patches. It even goes through spell immunity, which paired with her ulti can spell doom for plenty of ambitious BKB carriers. Fighting her  directly in her ugly face is an even worse of an idea than it used to be.
She has lost an amount of her total movement speed, which could really skyrocket. I believe that was the main idea though. As I said, her being fast provided mainly a getaway tool, and you didn’t really want to run away with your ulti on, but rather, to stand and fight. Thus, her movement speed was nerfed to a reasonable level and she got the HP drain and slow, to achieve just that.
The general idea seems to be in the correct direction. As amazing as it all sounds however, keep in mind that she can still be kited around easily, as a simple Force Staff is enough to break her link and her slow. She can be a powerhouse, mainly if assisted with decent crowd control by her allies to prevent those filthy rats from running away. Personally, I like this change; a decent buff, without taking her weaknesses away completely, unlike a certain blonde, white-robed wizard I could name.