X0202 SCHEDULE 13G 0001535978 XXXXXXXX LIVE Class A Ordinary Shares, par value $0.0001 per share 03/31/2026 0002094265 Mountain Lake Acquisition Corp. II G6301L109 930 Tahoe Blvd, STE 802 PMB 45 Incline Village NV 89451 Rule 13d-1(b) Adage Capital Management, L.P. DE 0.00 2100000.00 0.00 2100000.00 2100000.00 N 5.68 IA PN Robert Atchinson X1 0.00 2100000.00 0.00 2100000.00 2100000.00 N 5.68 HC IN Phillip Gross X1 0.00 2100000.00 0.00 2100000.00 2100000.00 N 5.68 HC IN Mountain Lake Acquisition Corp. II 930 Tahoe Blvd, STE 802 PMB 45, Incline Village, NV 89451 This statement is filed by: (i) Adage Capital Management, L.P., a Delaware limited partnership ("ACM"), as the investment manager of Adage Capital Partners, L.P., a Delaware limited partnership ("ACP"), with respect to the Class A Ordinary Shares, par value $0.0001 per share ("Class A Ordinary Shares") of Mountain Lake Acquisition Corp. II, a Cayman Islands exempted company (the "Company") directly held by ACP; (ii) Robert Atchinson ("Mr. Atchinson"), as (1) managing member of Adage Capital Advisors, L.L.C., a limited liability company organized under the laws of the State of Delaware ("ACA"), managing member of Adage Capital Partners GP, L.L.C., a limited liability company organized under the laws of the State of Delaware ("ACPGP"), general partner of ACP, and (2) managing member of Adage Capital Partners, L.L.C., a Delaware limited liability company ("ACPLLC"), general partner of ACM, with respect to the Class A Ordinary Shares directly held by ACP; and (iii) Phillip Gross ("Mr. Gross"), as (1) managing member of ACA, managing member of ACPGP, and (2) managing member of ACPLLC, general partner of ACM, with respect to the Class A Ordinary Shares directly held by ACP. The foregoing persons are hereinafter sometimes collectively referred to as the "Reporting Persons." Any disclosures herein with respect to persons other than the Reporting Persons are made on information and belief after making inquiry to the appropriate party. The filing of this statement should not be construed in and of itself as an admission by any Reporting Person as to beneficial ownership of the securities reported herein. The address of the business office of each of the Reporting Persons is 200 Clarendon Street, 52nd Floor, Boston, Massachusetts 02116. ACM is a limited partnership organized under the laws of the State of Delaware. Messrs. Gross and Atchinson are citizens of the United States. N IA HC The information required by Item 4(a) is set forth in Row 9 of the cover page for each of the Reporting Persons and is incorporated herein by reference. The percentage set forth in this Schedule 13G is calculated based upon an aggregate of 36,980,000 Class A Ordinary Shares outstanding as of March 20, 2026, as reported in the Company's Annual Report on Form 10-K for the year ended December 31, 2025, filed with the Securities and Exchange Commission on March 20, 2026. 5.68% The information required by Item 4(c)(i) is set forth in Row 5 of the cover page for each of the Reporting Persons and is incorporated herein by reference. The information required by Item 4(c)(ii) is set forth in Row 6 of the cover page for each of the Reporting Persons and is incorporated herein by reference. The information required by Item 4(c)(iii) is set forth in Row 7 of the cover page for each of the Reporting Persons and is incorporated herein by reference. Y N See Item 2(a). Y Y Y N By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were acquired and are held in the ordinary course of business and were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect, other than activities solely in connection with a nomination under ?? 240.14a-11. Exhibit 99.1: Joint Filing Agreement Adage Capital Management, L.P. /s/ Robert Atchinson 05/13/2026 Robert Atchinson /s/ Robert Atchinson 05/13/2026 Phillip Gross /s/ Phillip Gross 05/13/2026