From the website:
Whether you’re a grizzled MOBA veteran, or you’re not quite sure what “mid lane” even means, Mega Bundles make it easier than ever to dive head-first into the Nexus! Each bundle contains 20 Heroes, and has its own unique theme. The best part? We’re giving you enough free gems to unlock the Mega Bundle of your choice once Heroes of the Storm 2.0 goes live in your region after April 25! Check out the Mega Bundles below to get a feel for which playstyle will suit you best, then take our quiz if you’re still not sure which bundle is right for you.
Limited time offer. Mega Bundles will be available following the launch of Heroes of the Storm 2.0 in your region, lasting until May 22, 2017. All players who log in to Heroes of the Storm during that timeframe will automatically receive 100 Gems, which can be used to permanently unlock one (1) Mega Bundle of their choosing. Choose wisely! Once you have unlocked one of the four Mega Bundles described above, the other three will become unavailable. (picked up by Trop)
To claim, install the game, start it up and go into the 'Collection' section and pick one of the bundles to purchase for 100 gems that is credited to your account. Be sure to claim your Mega Bundle before opening loot boxes. Some loot boxes will give you a hero. The Mega Bundles won't give you shards if there's a duplicate but the loot boxes will.
Some facts for those deciding on the various Mega Bundles and want to get the most value based on the in game gold currency. The current gold value of the packs are:
Assassin - 145,000
Flex - 123,000
Support and Specialist - 131,000
Tanks and Bruisers - 141,000
Assassin and T&B has the most 10k gold heroes at 8. Flex has the least at 6.
What I did for my main account (which I plan on obtaining all heroes on) was work out the gold value of the heroes I own in those packs and subtracted these values from the above totals to determine the net gold value of those packs for myself. Current gold cost per character here:
http://heroesofthestorm.gamepedia.com/Hero_Prices
Of course, if you just want a bunch of free heroes to play the game with from time to time, pick the pack that has the heroes that suits you the most. For anyone farming daily quests, I'd also consider a pack with Abathur… pretty much the only hero you can set up and go afk in an AI game.
UPDATE: Launch event is 11am PDT 25 April which is 4am AEST 26 April.
UPDATE 2: Anyone just starting out and wanting to maximise the heroes you get shouldn't get the Assassin pack. Raynor and Sylvanas can be picked up for free via a 'Recruit a Friend' promo link: https://us.battle.net/account/en/creation/recruit.html?rid=L… (link corrected). The Flex bundle still has Sylvanas but if S&S or T&B isn't your playstyle, Flex will still let you get Raynor for free, though he's a 2k gold hero and easy to get.
UPDATE 3:Nexus Challenge 2.0 details (Youtube vid) - Basically mounts, cosmetics and loot boxes for HOTS and OW over 4 weeks.
nice i just installed it yesterday!